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.