When do you know when your tubes are going bad

Find a tube tester and test them.

When their grades start to fall, they stay out all night, you don't know their friends, they become withdrawn and sullen, or when the police call to come get them.

Alternately, a good tube tester.

If it's been sitting for years unpowered/unused - the first thing i would suspect is that a electrolytic cap(s) has gone bad.But you know - everyone's telling you to test the tubes for a good reason - cheap-easy - and real easy to fix if some tubes have gone bad - hope that's it.Unfortunatly tubes don't tend to go bad in storage (hence NOS tube prices)-electrolytics will last longer in use than they will in storage (hence no NOS electrolytic capacitors at any price).
Very carefully -and don't touch anything (serious electric shock hazard -even if not plugged into AC power) - take the covers off and using a good flashlight - inspect the base and ends of the capacitors.If any of them are bulged and especially if leaking - they've gone bad and need to be replaced.

I agree with stonedeaf. Unless it had this problem when you placed it in hibernation, it is probably not the tubes. When an amp hasn't been turned on for a while, it probably should be powered up slowly with a variac.