How do you know your tubes are beginning to fail?

How do you know that your music is suffering from tube deteriration? Do you bring you little babies to the tube Doctor to be tested? I would not know what to listen for. Also, when one of my driver tubes goes, time to replace all of them? 845s? Thanks in advance.
your stereo will sound 'sleepy' less dynamic with less detail and impact

check the archives as this has been posted several times
or it will get micrphonic and you will hear thru the speakers if you tap the unit or knobs.... or the quickest way to tell is if the tube goes and you hear a tornado approaching on a song that doesnt have a tornado in it.
mesaboogie, now that's something I can sink my ears into. great analogy