I cant tell if my cats are female or male...?

ok i know it sounds weird. i got two sibling cats a while ago and i was told one was a boy, the other a girl. but i see that their gentiles look the same! and we cant tell if they look like boys or girls. it looks too short to be a penis and balls but then again i dont know because ive never had a male cat and ive never looked there anyway lol is there any easy way of telling?