Saying "just use common sense" is very pat and appealing in cases where we have direct apprehension about the domain in question- such as in plane geometry- but in cases where we have no direct access to a question aside from experiment and abstract reasoning (such as deep history or physics,) how do you distinguish "common sense" from simple conservatism?

I'd imagine there's plenty of possible reasons pre-Clovis Americans wouldn't have been as numerous or left as much evidence as Clovis-people did; most obviously, that the continent was much colder, and therefore supportive of much smaller populations, than it was in the Clovis culture era- particularly in the northern regions, where it's often objected that they "should" have left remains. Obviously, this isn't an argument that there were pre-Clovis settlers, but an example of why the idea can't be objected out-of-hand through "common sense."