In my admittedly limited experience, the best way to tell if a capacitor is on its way out is to remove/replace it if you are uncertain if it is leaking or not.

Often other issues that crop up on the computer are due to the capacitors being faulty and/or affecting other components with their leakage, so that's one way to tell, but the problem isn't always replicated the same way for everybody from what I have observed. An exception might be with the SE and other compacts that get tell-tale issues with the display.

Sadly, this process is really just trial and error, though I suspect if your computer dates from 1994 or earlier, a cap replacement is probably overdue.