Going way, way back to my first years in this field, the primary indication of a gassy tube is the milky white color of the glass envelope, if glass of course. This would be indicative of lowered emission, which will impact gain negatively. But this is from my memory of burning my fingers on many a tube too many decades ago. If the tube is all metal such as the 4X150 & 4X250, transmitter finals used in HF up to UHF, then one must take the time to properly troubleshoot an issue and insure the input drive to the fianls is within nominal levels. Or that the final stage is tuned correctly and with a good match to the antenna.