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.