Actually, anyone with any real understanding of human skeletal structure could tell on a glance that Sans is not human!

Mainly because while a lot of him resembles a human skeleton, his skull is somewhat, uh, stylized, and skulls can't wink. Skeletons can't drink, eat, or sweat, either and Sans manages all of these, too.

So knock yourself out, pal! In my permissions post there's some info about what magically inclined people would likely detect, too.