Practically speaking, you should only replace vacuum tubes in an audio amplifier when you start to notice changes in the sound quality. Usually the tone will become "dull", and transients will seem to be blunted. Also, the gain of the amplifier will decrease noticeably. This is usually enough of a warning for vacuum tube replacement. If the user has very stringent requirements for observing vacuum tube weakening, the best way to check vacuum tubes is with a proper mutual-conductance-style tube tester. These are still available on the used market; though new ones have not been manufactured in many years. One tester is being manufactured today, the Maxi-Matcher. It is suitable for testing 6L6, EL34, 6550 and EL84 types. If you cannot get your own vacuum tube tester, speak to a service technician for his recommendations.
Eric Barbour, Senior Editor VTV METASONIX