They saw her face.
Hail to the king, baby.

Is that what you would have done?

This message has been deleted.

She looked like a girl to me. Her facial features are female. Her cheekbones, chin, eyes, nose all typically female. You shouldn't automatically associate short hair with being male.

This message has been deleted.

This message has been deleted.

pre puberty that's true as male and female bodies are almost identical, including bone density, breast tissue and testosterone levels.
But by 9 or 10 the difference becomes more than social identifiers (clothes, hair and so on) because body shapes change.

At a very young age, perhaps not, but by 11 or 12 years old, the distinction starts becoming apparent.

I don't think that applies to kids.

Up close and in person, you can usually tell. Male and female skull structure have significant differences that most people will pick up on, even if they're not sure what they're looking for. One of the strongest indicators is that an average male skull has a more prominent eyebrow ridge than a female skull.

I think I missed how the kids figured out El was a girl. I thought she was a boy until they referred to her as a female. Did they see under her skirt?

How did the guy at the diner and the boys know she was a girl??
That was my first question, too. A lot of the posters here are claiming that it's obvious but I couldn't have picked that up myself just from Eleven's facial features.
I suppose girls have different body language than boys. May be the way Benny figured it out was that if it had been a delinquent boy, he would have reacted differently to being forcefully restrained by Benny. May be he would have struggled more or tried to bite.
Eleven just looked scared and didn't put up a fight, which is a more "feminine" response. That probably led Benny to realise the truth.

no she looked pretty

She does not in anyway look feminine. Even when she put on the wig she looked like a boy in drag.

This message has been deleted.

Boys and girls have differently shaped faces. Millie Bobbie Brown has delicate features.
Did anybody have trouble telling that Charlize Theron was a woman in Mad Max? besides short hair, they taped her breasts, but no mistaking she was a woman. Unless u think the grease on her forehead is what made her look feminene.
But there are significant differences between an adult woman who has a shaved head and a pre-teen child with short hair. I have posted my thoughts below on how Benny understood that Eleven is a girl (I put it down to body language) but I couldn't have picked it up from her facial features alone.

Boys and girls have differently shaped faces. Millie Bobbie Brown has delicate features.
Did anybody have trouble telling that Charlize Theron was a woman in Mad Max? besides short hair, they taped her breasts, but no mistaking she was a woman. Unless u think the grease on her forehead is what made her look feminene.

Do I really have to say it
Breast Buds

This message has been deleted.

They actually didn't at first. Only when getting a close look did they realize. Benny, the restaurant guy, actually calls her "boy" but then turns her around and realizes she's a girl. Benny then says "she speaks" later on when they're sitting eating at the table.
http://www.imdb.com/board/14574334/board/thread/259516937?d=260073145#260073145

I'm something new entirely. With my own set of rules. I'm Dexter. Boo.

Sometimes boys look feminine until they hit puberty, and with the ages, I think it would be hard to determine if Eleven was a boy or girl. Even the guy who owned that diner in the beginning thought she was a boy at first. I personally don't think they would have known, so that part did seem a little odd to me.

It may be easy to tell

Yeah you're right I forgot about that guy from the diner who said it was a boy. I was just thinking that most people would assume it was a boyunless they really looked at her up close. I agree she does have a very feminine soft face but my initial thought was a boy.

didn't the one man (one of the patrons from the diner) Hopper was questioning say that they thought the youngster who Benny saw eating/stealing the food was a boy but that he/they (the diner patrons) didn't get a good look?
IMO when you look at Eleven it's very clear she was a female.