Here’s a great tip from the 6/23/10 ARRL Contest Update:
Uh-oh! You dutifully cleaned off all of your vacuum tubes to make them shiny and the numbers came off with the dirt! Pat AA6EG saves the day – “Put the tube in the refrigerator, cool it down, then bring it into a warm room temperature environment. There will be moisture condensing on the glass envelope, sometimes leav[ing] a readable outline of the tube numbers.”
If you have any tubes at all in your junkbox (mine’s more like a junk closet), you’ll undoubtedly have some that you can’t identify.