In the future how do I tell if my mollie fish is pregnant?

The female will have a black patch at the back where the babies come out of ,sometimes she may keep going to the top as she will be running out of breath alot but sometimes we dont notice and she may have a square belly at the front also if the male hangs around the female alot (underneath her)then that means she is ready to give birth soon because as soon as the fri(babies) are born then he will try and eat them ,so if you see any of them signs quickly buy a nursery tank and pop her in it.