@Wick Y'know I didn't even think about that.
We have three options:
1. Alexis is just so badass she can be a monster hunter and identify monsters even though she is unable to see through their disguises until they physically turn into their monster forms.
2. The Grimms were able to give her the ability to see monsters through some kind of blood pact or magic.
3. We make it so monsters give tell tale signs and that's what people focus on to see their true forms.
In all honesty, I really like the first one. It gives her this dimension of being an even better hunter because she doesn't have any genetic advantage, she really is just
that good at hunting. It also gives her another reason to be all about up in Elias' face for looking down at her (as if she doesn't already have enough), and it's something she can throw in his face.