when i need to know if a 9v battery is still good i touch my tongue to it. i'm guessing you could do the same to the tubes while they're in the amp. i'm going to test mine right now.
How can you tell when tubes are shot?
I have an ARC REF1 and ARC REF Phono Preamp as well as assorted solid state gear. I leave everything on 24/7 and am wondering if the tubes just quit working (ie: get really noisy) or whether there is a gradual decline in their performance? ARC makes recommendations on who long they should last but I have been running the REF1 steady for over 18 months and haven't noticed anything.