I'm not being facetious here, but asking this question very sincerely. What kind of evidence do you think would be sufficient? Since skinwalkers are just people, I'm guessing that evidence of their activities is what you would be looking for, correct? So the presence of certain herbs or poisons their house, maybe? Or would it have to be catching them red-handed trying to scare or hurt someone? Or are you specifically talking about the belief that they can shape-shift, and it's the shape-shifting that you would want evidence of? This now has me very curious if anyone has ever been legally prosecuted for this practice, like they have been for killing a person that had turned wendigo (although in that situation it was the person who killed the alleged wendigo who was prosecuted, and not the wendigo).