It would be very desirable if you could check the filament current at rated voltage because most tubes have multi-strand filaments, and one or two strands could be open without a piece breaking off and shorting. The tube could even operate with a strand or two open. Because of fragility, a filament can be broken without evidence of damage to the carton. Filaments are almost as brittle as a burned match stick. Grids are not much stronger.
Bad connections can cause trouble.
We frequently see evidence of this on duds that come in for reprocessing. Keep contact fingerstocks clean and tight against tubes. Wobbling tubes in and out of the sockets can bend and loosen the fingers. If fingers have lost springiness from overheating, replace promptly. Look for discoloration.