The female guppy may get very still before dropping the babies. You can put her in the breeder box/net a week before you expect birth. It's hard to say whether or not she will go into stress. They all react differently.
If you have reservations about using the breeder box/net, another option is to use a tank separator and place the female guppy on one half of the tank. Place some breeder grass for the babies to go into on the same side of the tank. The baby guppies will hide in the breeder grass. Net the babies and put them into the breeder box and then remove the tank separator. This may be less stressful for the female since she would have more space than if you were to put her into a breeder box. I've had success using both methods.
I'm not sure about when you can tell platies are pregnant. But if you watch the dark spot on the back of the silver sack inside a guppy, it will start to grow, showing she is pregnant. If you keep watching, you should notice a change in size of the dark patch in a few days. for her belly to grow it will probably be 2-3 weeks.
my little lady platy is pregnant and I'm sure she's about to pop - I can't see any darkness around her belly but she's very round, eating a fair bit more and she's got that "pregnant waddle". Her scales have sort of popped out (not off tho!) because she's so fat!
I'm looking forward to seeing the babies.. ;D