checking the rafters
sounds like a mess. but i think you must be misunderstanding things.
there is no way that asphalt shingles can be nailed on over just rafters. i can't imagine anyone doing this for any reason. if it is the case for some crazy reason, i wouldn't even consider buying the house, as the rest of it is probably in the same funky condition.
it's possible you've got "skip sheathing", cedar shakes, and then asphalt on top of that, which is common.
get up into the attic with a flashlight and take a "common sense" look at things.
does it look dry? do the rafters look sound? do you see sheathing on the roof? plywood? individual boards spaced a few inches apart?
and yes, a carpenter can assess this area just as well as a home inspector.
if your rafters are sound and the roofing material is not, it needs a new roof. if it's bad enough, it will need new sheathing also, which depending on square footage is expensive, but if you've got moderate skills not too hard to DIY.