Amp Re-cap

How do you know when an amp is ready for a re cap ?
Remove the cover and inspect to see if any are leaking. There will be a discolored area on the board beneath the base. If they are not visibly leaking, they still might have lost their specs which could be determined by testing.

Of course loss of SQ might indicate an issue as well.

There was a thread recently in which several members stated that caps can last 30 or more years if the piece is being frequently used. Long periods of disuse are hard on caps.
The half life is about 20-25 years, IOW about half of them will be needing replacement after about that time. Low temperatures during operation and frequent use both help extend the life. If in doubt, change the parts.

Hum in the amp or preamp can be an indication. Bulging caps, or safety plugs that are bulging are also good indicators.