You don't need to test tubes to see how they are performing unless you have a SOTA system and are anal about maintaining that sound level.
First, I agree with Eldartford, except it can be a PITA to put in new power tubes, rebias them etc. Small tubes is no big deal. One of the upsides to doing this is the friction on the pins on the tubes with the clips in the sockets will help clean them up.
Signs of tube demise is usually signalled by;
1) Noise (most often heard in the small tubes);
2)A dulling of tone. The latter is hard because it comes on very slowly, but IMHO as long as you still love the sound don't worry about it; and,
3) Make a note (tape it on the back of the amp) identifying when you last changed tube sets. Depending on how hard you are driving power tubes you can expect 1500 to 3000 hours. Small tubes are usually good for anywhere from 3000 to 10000 hours depending upon amp design. You could tet them if you had a tester, or you could then plug in a new set to see what you are missing, if anything.
Hope that helps a bit.