---First off, he's a Grimm. The group that attacks and kills Grimms are the Reapers. Hence, the antique/fantasy scythe that they use as their weapon of choice. Second, my theory is the simplest explanation. When he recognizes them, it's all over his face. That recognition shows that he can see through their "disguise". This is one, if not all, of the Grimms' abilities.---
--No way, They have to see him to other creatures can recognize them and even some humans can accidently thats a really impratical. I think he probably has a face to that they just as a show havent figured how to show i think they will def have it soon.
But please tell me this people: why can only some of the "creatures" recognize that Nick is a Grimm? Some r so shocked when they first see him, that he's a real Grimm and that Grimm's exist at all, and others have full conversations with him and walk away seeming to have no clue that they just spoke with a Grimm.......I'm very confused about that one question. Thanks
... it says in this wiki that the creatures can see Nick is a Grimm only when the creatures show their true face. They only show their true face when they are nervous, lose their concentration, etc. So when Nick is talking to the creature, he sees them as human at first. Then, in a moment of nervousness or whatever, the creature accidentally lets his true face show through. Then, and only then, can the Wesen see he is a Grimm. That is why the creatures always recognize he's a Grimm AFTER Nick sees that they are a creature, not the other way around. Creatures can also show their true faces on purpose, like to show other creatures that they are "one of them".