yekke2: There are examples where the Gemorah talks about non-jews that have a Bris Milah, this is one of them.
If I were cheeky, I would say Pashut (Al Pi Rav Chaim (which probably isn’t true anyway)) that one could tell by the extra tooth that Yidden have and non-Jews don’t. Even if the teeth fall out, the root holes are still there. 🙂