"So how do you play it off without it looking like you couldn't tell what she was?"
You can do the elbow test in a playful, teasing manner. Practice it first with a few women who are unquestionably female. Get them to do it first before you explain why, and they'll probably laugh when you explain after.
You could make it a challenge. "Hey, I bet my elbows bend farther than yours - here [bend her arm gently] - oh, crap, looks like you beat me!"
In fact, this could make a good excuse for touching any woman you're flirting with.
In the case where "she" fails the test, you could quickly excuse yourself with a "hey, I forgot, I need to find my friends" and step away.