Always look at the clutch and flywheel covers as well as the frame above the foot pegs for ware. In general, the more paint that is worn off the frame/covers, the harder it has been riden. Frames don’t usually crack unless raced or you’re buying it from a big dude. But check everything over, I’ve seen them crack all over. Frames get bent in the front from hitting stuff. Stock steering stems bend pretty easy too and can be hard to detect sometimes. You can always pull the oil filter and check it out too.
400ex’s are pretty bulletproof , easy to work on, and there’s tons of aftermarket support. Great starter sport quad. If it’s stock, you really can’t go wrong for the price.