The opening just under the tail is the anus. Below the anus is the genital opening which is round in males and is a vertical slit in females. In kittens of similar size, the distance between the anus and the genital opening is greater in the male than the female.
one on the right is female, the other could be male but because it's upside down, hard to tell. The key is in the space between the genitalia and the anus. The female will appear to be closer to the anus. The male has more of a space. I would google that for pictures and you'll be able to tell.