AS angel said they will "square" off they will also develop a very dark spot on their bodies. This is refered to as Gravid

it's actually the eyes of the baby fry inside of them.
I've had guppies in one way, shape, or form since I was a kid. I've seen females give birth to a dozen and call it quits, I've seen other females give birth to 30+ over a day or so.
Keep an eye on her and if you really want to see something dim/turn off the lights in the tank leaving your room lights on, if you're lucky you'll see a baby drop and start swimming before it hits the bottom....

Love seeing live births