When should caps be replaced?


I hear we should replace old caps in vintage equipment. But when and what are the symptoms of bad caps in vintage equipment? Other than oil leaking, what are audible symptoms?
mfdamon
My position is to replace caps that fail to meet their capacitance spec on a good multimeter or dedicated capacitance meter, of course if you see bulging or leakage on electrolytic capacitors, go ahead and replace them without thinking twice. 
I was wondering if there is a audible reference as to failure?
For the power supply cap gone bad, you will hear a 120 hz hum;

Crackling and popping noises from the speaker can be bad electrolytic caps in the tone circuit (balance, bass, midrange, treble);

A bad electrolytic cap on the feedback circuit can produce lower volume or higher distortion at moderate to loud volume setting in one channel.

As an aside, I took out of storage my 40-year old Marantz 235 receiver that hasn't been powered up in over ten years. The 10,000 uF power supply caps measured 9,600 uF with 0,02 ohm ESR. Practically like new. But it did crackle and pop a bit so I left it powered on over a variac at 20 volts for 48 hours to reform the caps and it sounded like the day I got it. Those vintage equipment give meaning to they don't build them like that anymore. 
@gs5556 after the caps were reformed, was there any change in measurements?
Power supply caps from the 1980's or earlier should be replaced by now. 

New caps (2000) to now I believe have about a 30 year lifespan. 

If you use higher than original temp caps you increase their usable life. 

Power supply caps are critical, as they can take a lot of other devices with them. 
Hi All

Newbie here. I read the thread and feel that my old 3240PE and 3020 A and 3020 B neither give the symptoms described. To my ear (which is very subjective I suppose :) ) they still sound sharp, clear and base is what I’m used to. So for my equipment, I feel leave well alone. You agree?
@ ahidler: my 1978 Son of Ampzilla has been in daily use (always on - never off) for the past 2+ years! Still sounds excellent! So your vintage NAD 3020 and 3240 are probably ok too! It's anybody's guess how long caps operated within their voltage and temperature rating will last! BTW, I used to own two 3020's long ago - great sounding integrated amps!
The Son has been playing music for 16 hours every day with no problems!