Actually, the best way to know she will drop soon is, she will get fatter than before. When a females gets pregnant, her gravid spot will get darker and her stomach will get big. But as the eggs finally get big enough for her to start dropping, she will get even bigger. I normally NEVER put my females in a breeding net or container, since she may get stressed out and abort or absorb the eggs, being in a confined space. Since I always monitor my tanks, I look for any fry, once I do, then I put her in the breeder net. When I know she's done, I move her into another tank so she can recuperate, so the males don't chase her around again.
I'm furtunate enough to breed my fish in my 37 gallon tank and put the fry in a breeder container. Then after the females are done giving birth, I move her into my 55 gallon tank with my angelfish, harlequin rasboras, clown loaches and the african butterfly. She's big enough not to get harassed by any other fish and there's plenty of hiding spots for here there, so she can rest. And no male guppies to worry about. I give her about a month before I move her back with the rest of the males.