ill bite.....in my opinion as much as the previous owner will let you get a way with. I would mostly say tires, srevice record, condition of all oils and a general looking over..you know paint wheels etc..etc..etc....a test ride is nice too if they let you......
how many miles are on it. asking them what kind of riding they did with it. local rides, expressway, weekend trips. I was told that bar hoping is hard on the engine. by this I mean starting then reving and riding away emediately without letting the oil get up into engine properly, and as the beers mutiply so does the reving and high rpm take offs.
First question would be how old a HS?
EVO or Twinkie?
Age makes a difference as well as riding style as mention above.
Maintanance records would be a must. Anybody who cares for their bike keeps them. If he has had the dealer service it, the VIN would give a record of service at the dealer.
Look at all the nuts and bolts. You can tell if they have been turned. Hard to put a wrench on a bolt and pull a cover without leaving evidence.
Check out the spoke wheels. If the rear wheel has any loose spokes and it is a low milage bike, it has been hotrodded.
Inspect the belt for rocks and any damage.
Make sure all the electronics function as designed.
If the seller ownes a trailer, ask him why and what he used it for. Rally bikes are low milage and only ridden up and down the drag, hard on the clutch but are generally well cared for.
Good luck on getting a great bike. Many are out there.