Well, only way to know for sure is to test it.
There's usually a getter flash (a thin film of barium or other reactive metal) deposited on the glass, that makes it apparent what condition it's in. Though, high power transmitter tubes were often made of adsorbent materials (graphite, tantalum, etc.) which don't indicate gas presence -- and also must be operated at elevated temperatures (the plate usually glows red hot) to reach useful gettering activity.
It doesn't look like there's a flash on that one, so you probably need an electrical test to tell.
Bring it up slowly, and check things as you go. Start by applying filament power, run it at somewhat reduced heat and no electrode voltage for some time, then bring it up to full filament voltage, and check that the current draw is within spec. If it's too low, it may be worn (filament evaporation, excess filament voltage in a previous lifetime..?!); if too high, it may be running cold due to convection cooling (i.e., lots of gas).
Then, test the electrodes at low power to see if the characteristics match up with the graphs, then operate it at rated power (60W at DC should be easy to achieve on that sucker!) for a while. You probably don't want to go straight to full power, because some gas will have diffused in over the years (and out from the materials), which could cause internal arcing and damage. Allowing everything to reach normal operating temperatures gives time for the getter to do its job and actually improve the vacuum.
As for the vacuum itself, glass is an incredibly good container, and the glass-metal seals are also top quality. The most likely invisible failure is an extremely small leak at a poorly made glass-metal seal -- but one so slow that it wasn't caught during manufacturing burn-in test. So, these are very rare indeed. Otherwise, if it doesn't have obvious visible damage (smashed envelope, cracking around seals), it's probably fine.
Tarnish on outside electrodes is normal from old age, not really a good sign, but nothing fatal; it might be worthwhile brightening the higher amperage connections, just to keep contact resistance low. But don't be hasty; if you use chemicals, you may cause more corrosion later on, or if you use abrasives, you might cut right through a low resistance or [somewhat-]corrosion-resistant layer.
Do make sure to operate it with the specified hardware -- cooling ventilation as needed, heat sinking plate connector, etc. Overheated seals are a prime cause of early (operational) failure!
Tim