hi WTM,
when a livebearer is about a drop. her vent will be open. it actually becomes nearly vertical ifyou observe closely. this is the method i use, if i see a platy like tat in the picture shown. i would put her in a trap, let her drop, collect the babies and release the mother. this wont stress the female too much as i only leave her in there for 1-2 days.
due date is near, keep an eye on her.
dropping soon, notice the gravid spot vent becomes vertical a bit.
she ready as her vent is very straight, put her in the trap, will be dropping in 1 -2 days.
good luck with your platties keeping.
regards
shiuh