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Here are some tips on checking/rejuvenating the CRT.
1. Try to find a fairly new ('70's-on) CRT tester/rejuvenator such as a B&K 467, 470, 480, 490 or a Sencore CR70, etc.). I wouldn't recommend any of the B&K units with a model number below 467 except for testing purposes only. The reason I say this is because the older CRT testers were rather brutal in the rejuvenation department and would often cause more harm than good. The newer testers have a more controlled rejuvenation function that will be less likely to ruin the CRT.
2. Let's assume that you've found a suitable tester and that the CRT test dead or very weak. Don't hit the rejuvenate button just yet. Try raising the filament voltage from the normal 6.3 volts to around 8 volts and let it cook for some time (at least 30 minutes). Then, return the filament voltage back to 6.3 and retest the tube. I've seen some CRT's that would die from sitting unused for decades and cooking them with a higher filament voltage will often bring them back. I wouldn't go any higher than 8 volts, as filament burn-out may result.
C. If cooking the tube fails to work, follow the directions that came with your tester and proceed to rejuvenate the tube. Most testers offer more than one level of rejuvenation and you'll want to start at the lowest level that will get the job done. If you can get the needle to move into the upper end of the "BAD" area or the lower end of the "GOOD" area, hold off on doing anything else for now as you don't want to push your luck and kill what life you have left in the CRT. I've seen many old B&W CRT's that still produced an acceptable picture while testing in the upper "BAD" portion of the scale.
D. If none of this fails to work, you can try a CRT brightener. This is a device that plugs in between the TV's CRT socket and the CRT in order to boost the filament voltage which will increase electron emission. I'm really not a big fan of CRT brighteners and only use them as a last resort. I'll add that I've seen a lot of CRT's with an open filament that had a brightener hanging off of it's neck.
E. If you find that the CRT filament is open (not lighting), there's not much you can do except for resoldering the pins on the neck of the CRT. Fortunately, an open filament is not that common.
When I get an old TV, the first thing I do is check the CRT to make sure that it's either good or it's something that can be brought back to life. No need in going to the time or expense of restoring the chassis if the CRT is shot. If the CRT is shot, you'll have to make a decision on rather to put the set to the side until a good used tube comes along, buy a new tube, or junk the set (which I hope you want do).
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