How will I know when a Krell KSA-80b needs to be recapped?

25...30 years, you don't need to really know when to recap. if it works, than you can decide weather you like the performance or not.

regardless, recapping old amp will have similar effect of the new strings on guitar -- fresh and crispy sound. also i would replace output transistors as they also tend to become noisy within same period of time.

You know when you "need" to recap the amp when:

1.  The caps failed and the amp doesn't work anymore.

2.  The caps show signs of imminent failure.  i.e. bulging capacitors, or sound quality is failing as a result of failing caps.

These are the only time one "needs" to recap the amp.

Any other cases are speculative theory.  If the amp is working, and there is no sign of imminent failure, then you don't "need" to do anything but sit and listen and enjoy.

Preventive maintenance is a good thing, but not absolute in this case.  You can budget for the cap replacement and do so at your leisure.  Or, listen for ten more years with no issues.

A lot depends on how hard the amp was driven (used) in the past. 

I would just enjoy