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
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!