Look and see if it has had a chain let go and break the case. Super common and a pain to fix right. Check to see if it was rolled/abused. Look at the condition of the plastics, seat, handle bars, levers, etc for cracks and bends. There are a few known weak spots on warrior frames that can bend, so look at those. (right behind the lower-rear a arms mount is a big one). Some other easy stuff is wheel/axle/swingarm/linkage bearings and balljoints. There isn't really anything that's super important to check, as these quads are pretty tough.