If you played it back in the olden days of course, since Wario Land II and Shake It make it pretty obvious in the opening cutscenes alone.
But if you played the original Wario Land, did you expect Syrup to be female? I kind of didn't, and the manuals might have been part of the reason why:
Yeah, they treated Syrup's gender like Samus' back in those days.
I never played Wario land before, Mario land is as close as it gets, though I'm looking for Wario land around pawn shops. Captain syrup, if I heard that name I'd guess it was some weird male.
But since I watched some videos showing Captain syrup I thought she looked ugly. Maybe she once was male...?
Hm, I played WLII first, so yeah, doesn't apply here. Anyway, I probably wouldn't even have known of her existence when playing the original first, as I've never read any GB, GBC or GBA manuals outside of this forum, and she isn't mentioned in the game until you actually see her.