If it's got little conkers under it's bum it would be a un-neutered boy, if there's a little baldy line where there may have been stitches it's probably a neutered boy. If there a line under the bum in an 'i' kind of shape then I'd say it was a girl. I'm sure other people can give better descriptions, but I'm sure a vet would be more than happy to take a look.
http://www.thepetcenter.com/exa/kitx.html
Perhaps you shouldnt click this link if you are eating or don't like looking at cats rear ends (which is completely natural).