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.