*rolls eyes*

This sexism in geek culture really annoys me.
We're meant to be the outcasts who were judged and generalized and should best relate to what it feels like.
Not turning around and doing the same to others.

And being perfectly honest it was a girl who had introduced me to D&D also, so a girl playing D&D holds zero shock factor for me.

As for your questions:

-How do you separate people with legit interests from fakers?
On a case by case basis you really can't 95% of the time. Unless if they clearly know nothing about what they claim to like (and all they seem to do is gloat about liking it).
Even then that's a bit unfair though, cause with me for example I dress like my profile pic shows at college a good amount of the time (found I liked the look :P) but haven't been able to see much of the 4th Doctor/Tom Baker yet, so those who see the hat and scarf and ask me a ton on that doctor specifically might walk away thinking of me as a fake.

-How do I feel on sexism in these communities?
See the first part of my post.

-Personal Experience?
Just one time in a D&D group where we were all starting a second campaign, and a new player was joining. When that player learned one of the players as a girl he and his family went "OMG! A GIRL IN D&D!" while me and the rest of the group just rolled our eyes at the sexist reaction.

-How much of this fake geek stuff is legit?
Somewhat depends on the area, but I say the overwhelming majority of the time this is just sexism.
People unable to understand that someone who is a girl could be a geek and/or someone could be a geek but a new geek (or new to a certain geek thing) and their knowledge may be limited so far.