measure the voltage from each terminal to the ground pin
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There is no + and - on AC current. That is the reason it is called "alternating". What you are referring to are the hot and neutral (or ground) leads. The black wire is hot and the neutral is white. The hot side should be the SHORTER slot on the plug. I always check with a multimeter just to make sure that no one previously screwed it up! Just check each slot, one at a time, grounding one probe to the wall plate screw. Happy Tunes!
The above posts are true.... The issue in residential wiring is the fact that your breaker box splits 230VAC. You can end up with an 180 degree out of phase condition if the electrician has wired outlets in the same room with breakers next to each other in the main circuit breaker panel. When measuring the outlets (assuming they are wired correctly) everything will appear to be OK when measured individually. But if you have an audio system with components hooked on outlets on 2 different breakers, you may want to check for the following condition. One of the easy ways is to put DVM meter leads between the 2 hot terminals of the outlets on separate circuits. If you have outlets wired out of phase, your meter reading will be 230VAC. If they are wired in phase, you should see 0VAC on the meter. For those who think I am crazy, go look in your electrical box and notice that your Air conditioning, Electric Range, dryer, and water heater all have breakers that are tied together. All of these appliances usually use 230V AC, unless they are gas appliances.
I know you're not crazy, but I'm wondering how this is going to affect the sonics if you happen to for example plug your source into one outlet box and your amp into another with opposite phase. Obviously, the neutrals and grounds should be at the same potential, and it isn't immediately obvious why the hot legs being 180'd would cause a problem. Does anyone have actual experience with this, or a sound theory as to why this would matter?
The only issue I have ever run across is transformer hum in some audio systems. The easy work around is to find the breakers in a panel an move them around so that your audio is on the same phase. If there is no problem, it may be desireable to have the amp on a different phase than the CD & preamp just to separate the power feed at the box. Sonically, I don't think there is an issue. I think the bigger issue is if you plug into a outlet wired backwards.