I have learned, quickly, to bite my tongue and wait. Usually Maeve will let it slide once or twice, and then she'll make eye contact with the person and say, "I'm not a boy." Then the poor, flustered person will hastily apologize (if it's an adult) or shrug noncommittally (if it's a kid) and move on.
My instinct, which I firmly squash, is to immediately say, Can't you see she's a girl? But nowadays I feel like she's allowed to present herself however she likes, and not be forced into narrow gender definitions.
I think the reason I want to react and correct is that as her mom, I deeply know and love who she is, and her femaleness is really a big part of that, despite the short hair and devil may care attitude. And I always want people to see ALL of her, immediately, even though that's not really possible, of course.
So do me a favor, if you're out with Maeve, let her be the boss of explaining herself to people. She does a great job, and it's really not up to us at this point.