Best case scenario would be to have your vet present to give him a basic check-up, failing that:
Questions: Has he colicked or foundered in the past? Is he a cribber? When was he last vetted, floated, wormed?
Touch him ALL over ( including ears, around the muzzle, on the belly) - do you feel odd lumps, bumps, see scars - if so, ask for an explanation. Look in his mouth - are his teeth reasonable for a horse his age? While you are feeling him all over, how are his manners about that? Does he have "no touch me" spots? Ask him for some basic movements on lead - backing, yielding, etc. Walk, trot on lead and see how he moves with no tack/rider (have someone else do it so you can watch him move to/away from you or have someone else be the watcher). Repeat this with tack on and again while mounted. Then do a lot of basic maneuvering at all speeds. Does he have a consistent and reliable stop? Is he responsive to your cues?