On the 87-92 models check inside the rectangular part of the swingarm that the carrier sits in. the lt 250 I bought had new axle bearings.. great. problem was that the carrier was broken into about 5 peices and the axle and bolts were the only thing holding the outside parts of the carrier in place! Found that about a week after I brought it home.
also if it runs, rev it up and look inside the pv window. see if the linkage actually moves the valve. this applies to the 87-92 models only.
Be wary of a bike with a 7 heat range plug in it. it could point to starting or other problems.  Some guys will throw in a hotter plug to try to make the bike easier to start. may start " first kick" like the ad says, but at the cost of a piston after you take it home.