When do you know when your tubes are going bad

I have a Music Reference RM-9 110W Dual Amplifier. It has been sitting in a box for a few years and dug it out tonight. My left channel is only running at about 10% as my right. I balanced the driver and tried to even out the bias but I am still getting a pulsation on the left with the bias maxed out. Should I change out my input tubes, output tubes, or put it back in th box? I really don't know much about tuning the amp but I know it was working fine when I boxed it. Any help would be appreciated.
When their grades start to fall, they stay out all night, you don't know their friends, they become withdrawn and sullen, or when the police call to come get them.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.