The female has a fan shaped anal fin and the male has a stick like thing (gonopodium) Here is a female and here is a male. When they have sex the male inserts the gonopodium into the female. This fertilizes the female and she may have 3-6 batches of fry from a single fertilization bacause female livebearers can store sperm for later fertilization. HTH
IF they already bred, the male will leave a trail of white stuff hanging from the female (only if they mated like a few minutes ago) this is the semen the female is tryin to absorb and the whole gonopodium doesnt go into the female the male just tries to squirt his semen into the female with his gonopodium so he gets his weewee to get as close as it can to the female b4 shooting his semen into her