You're so right. Most of the time they can 'tell' is because that person doesn't look like a stereotype of their gender. If a cis, straight woman has short hair, or works out, or has a more pronounced jawline, she's immediately trans, and is 'putting women in danger'. If a cis, straight man is more on the skinny side + short, or has 'womens' hobbies, he's also immediately trans and 'not a real man'. Like seriously both those people are cis you cannot tell someone is trans by looking at them 💀