Well, it can be kind of difficult outside of breeding, and it's best one you have both a male and a female to compare them, males generally have proportionally shorter tails than females. During breeding season, or after being settled in a tank for a while (for my male it took three months), the male develops a white to blue sheen on his tail and a swollen and roundish cloaca. Females have somewhat triangular cloacas and during breeding they become more volcano shaped. I believe one of the members here made a paramesotriton sexing power point, which would show you what it should look like.
