Have to play it by ear really. Try to avoid obvious damage, large knocks, broken dials and lots of rust are generally worth avoiding. If there are claims like "one speaker not working", it's probably been through over work so you don't know what state the working speakers are actually in (but on saying I had a guitar cab which had that problem, because of which it went pretty cheap, the connector had simply slid off the rear of the speaker, a quick crimp later and it was sorted).
If you can, try and try the amp before buying. Just to play it for a few minutes to see if there is anything clearly wrong (crackling, static, volume all over the shop etc).
I'm sure others will chime in with other points to keep your eyes open for.
If you can, try and try the amp before buying. Just to play it for a few minutes to see if there is anything clearly wrong (crackling, static, volume all over the shop etc).
I'm sure others will chime in with other points to keep your eyes open for.