Try swapping two of the cathodes over, unsolder the actual pins of the CRT socket (obviously green and one of the others) and link them across to each other. If the CRT is still green it's the CRT, if it's the other colour it's the drive.
I remeber trying this, but by this time the green had turned to a rather muddy colour which mean't it was too hard to tell.
I shall maybe try again later and double check :roll:
PS. I assume letting the cathodes float isn't a very good test for internal leakage then? I have also heard that some CRT's can be damaged this way. can you elaberate on this please Nigel?