How can you tell if a capacitor needs replacing?

My Vox Galaxie amplifier (1960's) is far too quiet.  There is some sound coming through, but not enough to be heard over a two person conversation in a library.  The tremolo effect works properly, and the light bulb lights up.  The fuse is good.  Any ideas on how to troubleshoot this?  It's too good looking of an amp (like an upside-down pyramid) to not use.  It's also heavier than my garage --weighs a ton!