check frame as mentioned, compression, trans fluid, coolant, bearings, etc. if the bike is been torn into there is no way to know the seller is being honest, just go with your gut. don't jump on a deal just because they say "only has a few hrs on it". if it's a non title bike call the local dmv with the vin.
and for all things holy. if it looks like it's spent it's whole life in a mud puddle walk away. water and rust takes it's toll on thing you can see just by glancing at it.