If it's 100% stock, it's probably still jetted real lean, especially a California model. look to see it the tail pipe is drilled out, or still restricted. Other than that, look for the standard stuff, leaky seals, wear on the covers from boots etc. Ask if there are any service records.
The XR400 is probably one of the most reliable dirt bikes ever built. If you buy it, two things - the spark plug is recessed deep in the head, and it's easy for small rocks to get knocked inside. Always use compressed air to blow out any debris before removing the plug, see if the factory plug wrench is included with the bike. Second, the oil filter cover is held on with two bolts that can strip real easy, don't ever overtorque them.
+1 on the swingarm bolt. No seller will let you remove it, but bring a stand and see if you can MOVE it. (Bring tools!...a breaker bar and sockets) It you can loosen the nut and rotate the bolt you are probably OK. If you can't budge it, that is a major deduct on the order of at least a couple of hundred bucks.
Put the bike on a stand and check the wheel and steering head bearings. Check for smoke at startup and any unusual loud ticks in the valves. If the bike is hot/warm when you go see it, come back in an hour when its cold. Then try starting it and check for smoke.
Other than that, just look for obvious abuse or neglect. I have two of these of the same year. They are claw hammers and about as complicated as a hammer too!