They smell off.

[He doesn't elaborate, but it's clear that's irritating him a little bit. Probably because even to a nose as attuned as Jack's is to Literally Everything, it would still be tough to identify magic made from a whole different world. Which, Rude.]

You couldn't tell by how anyone is acting, though.

All these herbivores galloping up to the watering hole for a free drink like they don't know the water's tainted.