here are some qustions you should ask your self to know if she is pregnat
1. is she fat
2.is thier a fish she could have mated with
3. is she eating alot-lets you know she could just be plain fat
4. is she lazy-lets you know its time
5. does she have a gravid spot-a dark spot above anal fin put scince your mollie is darkly collered its probanly not visable
dont put her in a breeding net to early it stresses her out and makes her give birth to early. if you have an extra tank that would be better so she haad room to move and relax. if so dont put any gravel in there fry will get stuck but put plant in their. after their born then you can move them to a breeding net but its easyer to let her give birth in seperate tank after she has droped her fry take her out becuase she will eat them!!! if you have any questions feel free to e-mail me
good luck