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

Great amp but getting old now over 14 years, depends on how many turn on cycles it’s had, but I would do them.
You can get say Nichicons same voltage and maybe even more micro farads for around $100 each, you need 4 of them. Make sure the diameter (to fit the mounting clamps at the bottom) and height (so it doesn’t touch the lid) is the same. If your handy and know + from - you could do them your self no soldering, everything is bolted, in around an hour.
http://www.audiotecnico.it/riparazioni/riparazioni_K/KRELL/KRELL_KSA80/immagini/KRELL_KSA80_power.JPG

Cheers George

PS: there are also a few other smaller electrolytic caps around the input/driver board that should be done also at the same time. 

Cheers George

If you hear a 60 hz hum and the amp sounds soft on the low end, then its time for replacement. If not -- and the amp sounds normal with no visible damage or bulging -- the caps are fine and whether you replace them at that point is up to you. Just be aware when you send in an old amp with crowded boards for recapping, other parts tend to get replaced that are in the way, which is a factor why these repairs run in the three figures.

14 years isn't too old yet if it's been always used.