How do you know when your capacitors need replacing on a vintage SS amp?

If the amp is over 35 years old, just do it.

Caps can bulge, you can have the plastic sleeve on them shrink a bit (an indication its been running warm) and on computer grade caps, there's a little rubber plug near the terminals that should be perfectly flat. If not, replace the cap.

If the amp hums it could be a sign as well. If you measure it, it won't meet distortion specs. And so on.

Thanks, y'all. I didn't think to try Youtube or even Google since i've gotten so much expert help here in the past.

my experience:

bought a vintage McIntosh Tube Tuner/Preamp mx110z made in 1964. Took bottom off, no repairs or alterations visible.

Played, sounded great.

What the heck, dropped it off at Audio Classics, they replaced 10 capacitors, and 15 resistors. Nothing 'blown' or 'dead', but they get 'out of spec' i.e. no longer +/_ 2%, old stuff +/- 5%

More importantly, they replaced all the old corroded jacks with new gold plated ones.

Sound better? I don't hear any improvement, but those new jacks give much more secure connections. Before, I was having intermittent imbalance, jiggled a cable, ok, next day ,,,

just do them.  Most of the old amps used crap capacitors anyway and you will improve your sound