I had a similar thi happen JUST yesterday.
I asked the guy for a male mouse (I have a very picky BP) and he goes, "Uh.. I can get you a mouse, but I can't guarantee it'll be a male." I asked why (I always get male mice on request) and he said because he didn't know the difference...
By the way, these are adult mice...so it REALLY shouldn't be too hard....
Needless to say, I rolled up my sleeves and got my mouse.