The biggest problem is that the magnets will “quench”, which is what happens when a superconducting electromagnet suddenly stops being superconducting.
There’s a lot of energy stored in that magnet, and when it quenches the energy all turns to heat in a very short time. Any remaining helium will flash boil, turning into an explosive expansion of gas, and the thermal shock will seriously damage the machine
They will have some kind of pressure relief valve, to let steam out and prevent an explosion. They only become dangerous if that valve isn’t working (assuming that whatever keeps the lid on is intact and still strong).
Look for damage around the seal between the pot and the lid, and look for damage to the clamp or latch which holds the lid down against that seal.
Then look at the valve. It’ll probably be a heavy object (such as a lump of metal) which sits on top of a hole of some sort, or it could possibly be something spring loaded. Either way, check that it moves freely.
After that the only additional thing you could do is a pressure test, where you basically deliberately overpressurise it and see if it explodes, but if you had the means to do that safely then you wouldn’t be asking for advice here so I don’t recommend it.