[kbmuri]

The silver or mirror-colored portion of the neck will turn white. That means air has chemically interacted with the getter material deposited on the inside of the neck. This is true of nearly all vacuum tubes, not just CRTs.

That's a "slow leak". Of course the obvious indicator of a CRT losing vacuum quickly is the face of the CRT will lose a lot of the phosphor. Usually a big ugly black spot in the center of the tube.

They'll also show no emission, but that's a side effect. Open filament or completely worn out cathode will also show no emission, even if vacuum was maintained. In the case of worn out cathode, that's when the Beltron Restorer comes into play. Usually it can fix that.