The most common cause of tube failure is open filaments. Evidence of a tube defect is often obvious when the filament is open in glass-envelope tubes. Also, when the tube becomes gassy or when arcing occurs between electrodes, you will probably have visual indication. Metal-encased tubes can be felt for warmth to determine if the heater is operating. You can tap a tube while it is operating in a circuit to reveal an aural indication of loose elements within the tube or microphonics, which are produced by loose elements.
Most tubes are extremely fragile and subject to damage during shipment.
When you replace a tube, never make the assumption that the new tube is good because it's new. You should always test tubes before installing them. However, in high-frequency circuits tube substitution should be carried out in a logical sequence. Replace tubes one at a time so that you can observe the effect of differences in interelectrode capacitance in the substituted tubes on tuned circuits. Under these conditions, using test equipment designed for testing the quality of a tube saves you valuable time.