“I know a man when I see one” has gotten so many cis women harassed because of transphobic bigots. Women with things like mustaches they don’t really feel like shaving every day.
My wife works with a woman like that. I haven’t talked to her about it because it’s none of my business, but I thought she was trans myself at first. I said to my wife that she must get harassed a lot as a trans woman with a flat chest and a mustache and my wife told me she wasn’t trans but gets harassed a lot anyway.
I try to be open-minded and accepting, but I made an assumption a cis woman was trans on sight anyway based on biases about what a woman “should” look like that I’ve been shown my whole life. I’m sure they’re no more expert than I am.