Cats are mamals. Lift the tail and look for cahoonies. If it has none, it is either female or neutered. Looking for the penis is easy enough if the cat allows you to flip it over. Unless it is your very own cat, good luck with that! See you in Emergency room. Males also have nipples, just like every other mammal, so  those are of no use when determining the gender of a mammal. Uhm, yah, most mamals actually don't have swollen mammaries unless they are breastfeeding little ones, so don't go there. Very young cats can be confusing, and a tomcat's penis can hide quite well. A bit of pressure just in front of the anus will expose any sign of male genitalia in kittens.