look for an side to side play in the rear axle to see if the bearings are shot.
look for puff of smoke out the ehhaust to tell if its have valve/guide problems.
watch out for really soft and bouncy shocks in the rear too.
overall they are rock solid machines but they are also older so wear and tear becomes a factor. i had one for years and they are lots of fun and real reliable. cheap to fix too. if you have any other? just ask.