[Arran]

Well some tube testers will actually have a test for gas, but one way to tell if you have a gassy tube on a yester without a gas test is that the tube will test unbelievably good on quality, sometimes almost pegging the meter. Sometimes a gassy tube will also have a purple glow when the B+ is applied, though it is hard to see on some tubes, I had a #47 tube that was like that. Recently I found a 6F6 that had gas, or a secondary emission issue, in the set it works fine for about five minutes, then the audio became distorted, the same set has a different issue that I haven't sorted yet with some sort of distortion.
Regards
Arran