I don't think it would ever come to me that "he" was a girl, but I'd still think something was up.

Probably

I tend to be rather good at spotting little things. Can't think of a decent example right now, but y'know.

She. Looks. Like. A. GIRL.

So I'd either figure it out, or my gaydar would be going wild.

^gaydar, nice.

i mean, she looks like a girl, she talks like a girl (yeah, she still somewhat sounds like a girl through her 'man voice'), she's tidy and clean... those soldiers were complete idiots.

It depends. XD In the beginning she was more obvious, but as the movie went on, she became somewhat less obvious.

Same as firegirl1515, if I didn't figure it out, my gaydar would be going wild. XD

Thing is, part of being in Shang's army would include being raised in that culture and time, to think women were all meek and just uselessly sweet. If someone tried it in this day and age I'd realise, but if I'd been a man who was raised in that culture while I'd absolutely be suspicious ('Ping' was a weirdo who couldn't get his own name right and seemed to be the only one insisting on a private tent. You'd have to be crazy not to be suspicious of that) I don't think woman would occur.
Me as I am know, I'd realise, but if I was a man raised in that age and culture, I don't think woman would be my suspicion. Or if it was, I'd probably dismiss it.