Having fixed and owned a few old skool amps, I haven't had any that the caps went bad on personally, but I have heard and read of it.
Here is what I have heard and used myself:
But when caps go bad, there is usually a physical indication of that. It will bulge or leak fluids, or simply explode. If that doesn't work, usual practice is to just replace it.
Let me say this, I've been in electronics for a lot (very lot) of years, and I never needed to test a cap. Either it looks bad, or it's not bad or I just replaced it.
Also, I have had better service from Older amps, particularly Zed made and US AMPS and RF amps. Whatever You do, don't buy any of the Rockford Fosgate amps like the 400aII series or 400.II if they are Brokenbecause they are hell to fix the MOSFETS on.
My personal favorite old US MADE AMPS are US AMPS AXTU hybrid tube amps with different tubes, US ACOUSTICS Zed made amps, McINTOSh, and the PPI art series amps. The Japanese made Alpine and SONY Mobile ES amps are also very good. It is honestly hard to go wrong with any of the old USA made amplifiers of good name makers. Watch out for weird connectors/plugs/crossover board-chips that may be missing or damaged as many of these are so obscure that although the amp may work great, there is not a way to hook it up anymore because the connectors are so rare. I have had the best luck with repairing US Acoustics amps and most other amps that were zed made. The retainers for the MOSFETS and other IC's are usually easy to service and the boards have a very clean design.
I don't think I ever repaired a ZED made amp that the failure was caused by a part, the failures were always from shorting out the speaker leads or being connected incorrectly or ran with no fuse or a shorted fuse (yes, I saw that twice). None ever had a failed Capacitor that I found.
CLIFFS NOTES: You can usually tell when a cap $hits the bed