User blog comment:That1Guy99/How come humans still don' know about wesen?/@comment-5355892-20130507132401

There are two reasons as to this -  at least to my understanding....if a Wesen is stressed out or upset he/she will woge out...and if he/she is angered he/she may woge as well. I assume that being a Grimm and all Nick can see them even if they don't want to be seen but i'm guessing everyone else is unaware of it.