The vagina, or more commonly referred to as the vulva (opening to the vagina) is in the groin area right in front of the anus. It looks like a V shape the kind of hangs down and when in estrus or "heat", this V area will swell and become more noticeable. Sometimes blood or a discharge will appear in this area as well during the heat cycle.
As for the male, it depends....if the neuter has recently taken place, the skin of the testicle sack will sometimes still hang down but will be empty. Eventually, the sack will shrink and is sometimes barely seen at all.