This is a very informative & great thread as Karelfd mentioned above. I also have to agree with Swampwalker that just because a tube exhibits microphonics does not indicate the tube is worn out. I have purchased brand new tubes that are microphonic in some circuits but are not microphonic in others. The use of tube dampers can greatly reduce or eliminate microphonics in most cases. I think I'm a little paranoid over purchasing gear that uses too many tubes and for that reason prefer a more simpler design.
If you don't have a tube tester the only other thing you can do is watch out for a sudden deteriation in sound or a drop in gain. I have had tube preamps where the gain suddenly drops and the tubes cease to function period with no harm and they sounded very good up to that point. I would think that one would want to exercise more caution here in regards to tube life with tube power amps since the sudden death of a worn out tube could cause circuit damage or in worst case scenario a small fire. So as mentioned above it is always prudent to have a spare set of tubes on hand. I don't know why most manufacturers of tube power amps don't include a electronic metering device that lets you know the amount of hours you have put on a new set of tubes.
If you don't have a tube tester the only other thing you can do is watch out for a sudden deteriation in sound or a drop in gain. I have had tube preamps where the gain suddenly drops and the tubes cease to function period with no harm and they sounded very good up to that point. I would think that one would want to exercise more caution here in regards to tube life with tube power amps since the sudden death of a worn out tube could cause circuit damage or in worst case scenario a small fire. So as mentioned above it is always prudent to have a spare set of tubes on hand. I don't know why most manufacturers of tube power amps don't include a electronic metering device that lets you know the amount of hours you have put on a new set of tubes.