Cop a feel. Just know, if you feel any male parts, you are gay.
Remember a guy in college who’s super-safe way of finding out in Thailand was to check if their arm could bend past the 180 degree mark at the elbow…apparently men can’t do it.
I have no idea if that’s true, or how you go about bending back a womans arm back in a bar, to find out if they’re a dude, but he was very proud of his method…
Admittedly, he got a girl pregnant out there, so he guessed right at least once!
“Excuse me madam, but do you happen to have a wiener on your person?”
“How’s it hangin’?”
Judging from what I’ve read here nearly a decade ago, there’s a flow chart if you want to take chances like this
1: take “her” (heh heh…right) home
2: have sex in the dark
3: wake up next day
4a: look over and see logalog (see 5a)
4b: look over and see kachooch (see 5b)
5a: kick him out, burn the mattress (and his clothes if you feel that strongly about it), then post about it here.
5b: :tup: Take pic, post about it here.
Offer her a free trip to Japan to take photos.
/NeverForget
Plat twist. She’s a cancer patient. 
Either of both, depending on the country.
I’ve actually been in this position before lol. All I did was say “Hey no offense, but are you male or female. My best friend, who I love to death is transgender and you remind me of her for some reason”.
Does it matter? Its 2014 you can’t discriminate based on gender. If it got a big booty just smash it from the back. Problem solved don’t ask don’t tell.
Paging Missing Person.
Rape her.
If he/she know Okama Kempo, then that’s a dead giveaway
wanna measure diccs?
easiest way to find out
if there’s grass on the field play ball!
Tell her you want her to fuck you in the ass. If she doesn’t need a strap-on…
If there’s a tree in the field… um… cut it down? 
Just ask.
This has to be a troll thread because some things are just ridiculously stupid. Stalk her on facebook and look at her mututal friends, see if she has any baby pictures.
or just fucking ask.
Unless shes asian or hispanic, I seriously doubt its thats hard to tell if shes transgendered.
Do you play stick or pad?
Grab her crotch, see if the family jewels are there or not.