As stated, check the chain and air filter, but also check in the tank for rust. If you have a borescope, use it and look inside. Ask for maintenance records. I always check the condition of the oil, and coolant. Check the wheels for play. Take a very careful look all over for missing fasteners. Check the clutch for freeplay adjustment. Check the tires for wear and care, but also check the date codes, if they are getting too far up in years they may be due for repalcement. All of those items can give a good indication of the level of care it received. Tell the seller you'd like to see it cold and to please not start the bike before you arive. Feel the engine when you get there, if it isn't completely cold you are likely dealing with someone who has something to hide. Cold starts can expose some basic issues like valve clearance problems or lack of compression. Also check the mileage against the maintenance schedule. If it is due for anything major you want to be aware and also potentially use that to help get the price down.
