Gravid spots don't always show on livebearers, when shes due she'll start to hide away near the bottom or top of the tank,normally by the heater or filter, another tell tale sign is if you have male mollies the will start to chase her nipping at her rear end..
If you have plenty of plant coverage let her have them in the tank, putting her to early in the trap may stress her out and she might abort the fry.
When my livebearers first dropped i managed to catch them with a turkey baster and put the fry in a trap,in time i got used to when they were due to drop within a day and would gently catch them,few hours later out the fry come,then she was back in the tank.
Its not good to keep them in a trap for more than 48hours.