It's hard to tell until the fish 'squares off'; that means her belly will develop a distinct 'boxy' shape with corners at the front. Once that happens, you can expect her to give birth within the next day or so.
Do bear in mind that, if you put the fish in any kind of breeding net or box, she may find it too small and be so stressed she might try and 'hold on' to the fry and not give birth when you expect. Our advice is always that the mother fish is best left in the main tank, and plenty of hiding places (live plants are best) provided for the fry.