Well your character:
1) has impressive mammaries
2) wears lady glasses (guy glasses tend to have thicker top sections than lower sections)
3) is average height (since nothing about her height has been mentioned) and therefore considerably too short to be a male at first guess
4) has a lady face (even if the top is magically obscured without disturbing her ability to aim down a rifle, the bottom of her face is still feminine)
5) has long hair

I do not see how assuming that she is in fact a she is metagaming.