Tube amps haven't very many places that can go wrong..they're extremely simple really. Just like any amp, I'd check for scratchy pots and overall that it sounds good and has plenty of volume. The only things that can go wrong and aren't maybe immediately noticeable would be bad tubes, bad capacitors and FX loop connections (sometimes on older amps you have to use a patch cable accross the loop if there's nothing there, they can cut out the sound thinking something's there when it's empty...I've had two amps do that, even my current Tech-21.
Turn it on and up...if it works and sounds good it's probably OK...it should be quiet without any weird 'pings' and rattles. Those would mean bad tubes somewhere...full tube job costs maybe ~60 bucks over here, dunno about there though. The caps are the only thing that could be expensive to fix and hard to know if they're still a-ok...sometimes they last 2 years sometimes 50.
Someone will probably give more pointers...
Dee
"When life's a biatch, be a horny dog"
Amps: Marshall JVM 410H w/ Plexi Cap mod, Choke Mod & Negative Feedback Removal mod, 4x12", Behringer GMX110, Amplitube 3/StealthPedal
Half a dozen custom built/bastardized guitars all with EMG's, mostly 85's, Ibanez Artwood acoustic & Yamaha SGR bass, Epiphone Prophecy SG, Vox Wah, Pitchblack tuner plus assorted pedals, rack gear etc. for home studio use.