How do you tell how dirty your power is?

If this has been covered I am sorry but I want to know how you can measure if your power is dirty and thus evaluate best course of action, I assume no power is perfect but some must have it much worse then others.
Does it sound better at different times of the day, or if more or less lights are turned on, the dishwasher, washer or dryer running?
Good question. I've often wondered the same thing. I've considered buying, say, the PS Audio P500, only to find that it's impact on my sound was negligible becasue I already had clean power.
Get a PS Audio Noise Harvester, they sell for $75 apiece (used). It plugs into your outlet only, no devices plug into it, and any noise in the line is used (consumed) to power a small light bulb. The more noise in the line the more it lights up so you can see when your refrigerator goes on or light dimmers are in use. The more devices you use, the better, it really lowers noise. THe noise harvester is not a meter so it will not give you a "numeric value".
Well, the technically inclined may have an answer. My answer ain't so inclined. Buy a conditioner;used. Buy one that can be sold for what you paid for it. Try it;if you like what you hear you are moving in the right direction. Further advice: don't expect to much from a cheap conditioner.I believe they,(cheap ones) give you this/but take away that.---Over the years I have owned at least 20 conditioners. I now use the Adept Response. Used, they run around 2k.In five minutes' time you will know just how dirty/or clean your power is.
Krell_man is on the right track. If your system sounds obviously better after midnight (2 am) on a weekday, power is likely an issue.
Plug in several ordinary low wattage light bulbs in as many sockets as are in the circuit you use for audio.add a few cheap multi A/C adapters and insert more low wattage ordinary light bulbs. (the light bulbs tend to dampen and eliminate A/C spikes, High frequency junk.
Now if your system sounds BETTER with all these light bulbs IN.... you need power conditioning. If it doesn't change... you probably do not. If it sounds WORSE.. you need a rewiring of your A/C mains
This is a VERY cheap test. You can plug in ordinary lamos etc, as long as they are common light bulbs.. (NOT Halogen, not compact flourescent)
Would power impurities show up on an oscilloscope? Can one be rented? Are you familiar with reading and operation of the test gear?
A scope is the only way to assess the non-60 Hz information on your ac. I doubt if anyone rents them, however. Don't be too certain that ac filtering, etc. while getting rid of some of the noise will sound better. Most such devices hurt the sound more than they help it.
TBG makes a good point. There are several types of power filtering, from transformers to chokes to capacitors to active re-generating devices. Each has its place and results in different trade-off's. Some can work together and others can't. Some work better for source components but not for amps. There's more to this science (yes, science, because each has a provable result on dirty power sources) than just buying something and saying "it helps" or "it makes things worse".

There's a lot posted about power conditioning on many forums. Do some searching and read up a bit. Then by all means, buy something used that makes sense and try it out. But just because that may not work doesn't mean that the correct type of power conditioning isn't needed in your room.

I didnt know the PS Audio product has a light that can tell you how it is working, I have a Blue Circle Noise gizmo but cant tell any benefit, thanks all.
A different way to consider the question is 'how can you tell how robust your equipment is?'. The better your components internal power supplies are designed, the less neccessary or detectable any type of ac power fiddling will be. I really like Elizabeth's note about using lights for a cheap sensitivity test, cept I'd go for high wattage (more current draw, less resistance, more dramatic effect).
One of the worst culprits, and very easy to assess, is fluctuating line voltage. It's a serious problem if you use a tube amp with manual bias. Just stick the probes of a simple volt/ohm meter into your wall outlet at different times of the day and see what it reads. If it's always low (118-ish) or fluctuates between 118 and 123, then you're going to need a regenerator like an Exactpower. No conditioner will help, but a regenerator in addition to holding a steady 120, will do everything else a conditioner will do but without ruining dynamics, etc.
Plugging in something like the PS Audio Harvester will give you visual confirmation that there is noise in your power supply.
If you turn off all appliaces in your home and notice that you are hearing more details which were previously absent you have audible confirmation that there is noise in your power supply.
The other audible difference is in the music which should in most recordings never all have the same volume i.e some instruments must be clearly softer than others.

If your audio system shares power with any other appliance chances are it will be "dirty". The often overlooked solution is to get a dedicated line from your distribution box to your audio system.

There are many "solutions" out there such as:
Regeneration - PS Audio
Balanced Power - BPT
Passive Filters - Shunyata
Transformers - RGPC
Using Shielded and/or Braided Power Cables

Find the ones that works for you.
Athipaul, what confirmation is there that lights flashing on the Harvester or noises coming from the Noise Sniffer show noise on your ac line? I agree with your basic advice that if you try a device and like it, why not use it. Again, apart from a dedicated line, most other corrections may harm as well as help ones sound. Often greater quiet comes with less dynamic range. Would it not be great were ones power pure 60 Hz only at 120 volts? Would it not be great were power supplies able to yield pure dc at the desired voltages regardless of what comes in on the power lines?
I say buy a PS Audio Powerplant Premier.

It has a distortion meter and gives you percentage distortion of raw power vs. after it has been regenerated. Usually around 3-5% distortion from the wall, and .3-.5% after regeneration. (one tenth of what it was)

This way you know not only how bad your power is, but what it sounds like cleaned up. PS gear has a 30 day money back, so if it's worhwhile on your system, you'll find a way to pay it off, and if the improvement was not substantial enough, send it back. All you lost was the shipping.
Thats a question you have to ask the PS Audio people. My understanding of how it works was if there was noise it would blink. I don't use the Harvester anymore as it stopped blinking the moment I plugged in an RGPC. With regards to dynamics, I only noticed the difference after I plugged in the RGPC.
Guys, I have seen what my ac looks like on a scope, and I have tried many, many devices that are intended to improve your power. The ultimate question is whether the device improves your sound. I worry that measures, such as that with the PS Audio Powerplant's meter, may only capture one aspect of the problems with your electrical service.

I had a Noise Sniffer and many of their active devices that you plugged into circuits that had high noise on them. In each instance when I plugged the device in the noise dropped. I did this until I exhausted the eight devices I had purchased. When I listened to my music, it was quieter and totally uninvolving. I began removing devices near my audio and found the sound improved with each I removed. I thought that surely those in remote locations would have only positive results, but I was wrong. I sold everything. What was the Noise Sniffer measuring, and why was removing it harmful to the life of the music?

I know fullwell that your experiences may differ from mine. I always advise people to let the new improvement settle in for a while and then remove it. If you lose the magic, reinstall it. If you regain the magic, sell the device. I have sold or tried to sell probably 40 such devices over a long career in audio.
You need the Clever Little AC Power Analyzer. It appears to be a set of plain old television rabbit ears but in reality accurately detects the quality of your ac power. Place it near the outlet in question for 12 hours and then very carefully move it outside to the location at which AC power enters your residence. Make sure the wind is blowing at least 20MPH. After 24 hours if the rabbit ears are facing north/south and it is an even numbered date, your power is fine. It they are facing east/west, your power is suspect. On odd numbered dates, the wind must be over 40MPH and the north/south and east/west positions must be reversed.

If the wind speed is such that the rabbit ears move, the use of magic pebbles to anchor them is suggested.

Let us know how this shakes out for you.
Bill, sounds kind of complicated, is this a service that can be provided by Audiofeil International?

It is far beyond my technical expertise.

However, there is another vendor advertising here far more qualified than me.

In fact he might even be an MD.
One aspect not mentioned is that the manufacturer designs and tests the equipement.
Do they test with "perfect sine wave power? Or the power coming out of the labs socket?
My adventures in power conditioning lead me to believe that some scuzz in the power IS NEEDED!! to sound the way the manufacturer intended.
Too clean and thin a sine wave and the sound becomes as said! "lifeless"
This is a difficult area to know just what the designer used for power: clean? dirty?
Elizabeth, is this tongue in cheek?
The harder you are,the dirtier it is.....unless your from Fort Apache or Queens..jhds7w9le,mnbs/.q926bswjmy5qG^&..sorry,just lost it there for a sec...
Audiofeil has it wrong! That's for RFI. lol
if you have a laptop, play some music through your system with it plugged in, then during a soft passage unplug it letting the battery power the computer. i can hear all the noise in my ac with the computer plugged in and it is dead silent with it running in battery. very simple test. btw, if you have a power conditioner(i do) make sure you don't plug the computer into that but directly into the wall.
I did try the laptop experiment and could not hear anything, it is not on same circuit.