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.

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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

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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.

The fact that all of the characters and most people on this message board realized she was a girl says something about YOUnot the actress.
I've seen several posts on forums where people thought she was a guy. Hell, even some of the adults in the show thought she was a guy.
Maybe you're just attracted to guys or butch girls. Ever thought of that?
I also thought Mike Wheeler was a scrawny middle aged woman for awhile.
jk90

39 of the last 45 Best Actress Oscar winners have been nude in films.
See how that works?

Brown does not have a "manly butch" face. It takes a special kind of person to look at her up close and not realize she's female. The fact that all of the characters and most people on this message board realized she was a girl says something about YOUnot the actress.

i think it's prejudice regarding short hair.
How the hell is that prejudice?
apparently woman can't have short hair without looking like a boy. and i think that is pathetic bullcrap.
Not at all, just as men can have long hair without looking like girls.
But there can be some confusion when girls have manly butch features (like Eleven). I thought it was a guy until someone said it was a girl.
jk90

In my opinion she did not look feminine at all. If the series had not specified her gender I would not have been able to tell. Lol. Mike had a more feminine face to me.

Ban short hair shaming!

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Eleven had a very feminine face in my opinion, it wasn't that hard

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How did the guy at the diner and the boys know she was a girl??
I'm still not convinced it was.
Or that Mike was a boy. He looked like some middle aged woman.
jk90

@OP
Girls are already well into puberty by age 12. Their hips are quite noticeably wider than boys by this age as well as their glutes being plumper. It's been like this since I dunno, forever?
By 13, a lot of girls in my jr. high already had adult b-cups and the 13 yr old boys weren't too shy to gawk at them either.

It's really easy for most people. Shave your head and walk into the nearest 7-11. Tell the clerk you will give him/her $100 if they can correctly guess your gender. 99.9% of the time you will lose $100.
He killed sixteen Czechoslovakians. Guy was an interior decorator.