How can you tell a cat is a female or male?

Same as you would any other animal. If you are unsure consult a vet or cat breeder.

The best way to tell what gender a cat is, it by looking underneath the base of its tail. A male cat that has not been neutered will have two furry testicles right below the anus, and a penis right below the testicles. A neutered male cat will not have visible testicles, but there will be a noticeable space between its anus and penis.

Spayed and intact female cats alike will have a genital opening (vulva) which is often seen as a tear-drop shape located right below the anus.

Please note that checking for nipples is not an accurate way of telling a cat's gender - male cats can have nipples too! They just have no function, much the same as human males have nipples but have no use for them.