There is a black spot just before the females tail. When this spot gets bigger you will know that she is pregnant. As this spot is almost half the size of her stomach she will be ready to give birth. This black spot is her womb. If you have males with her in the tank you will see that they start to attacking her. They know that she is about to give birth and see this as a threat. Hopefully just befor this stage you have but her in a breeding tank. As the other fish will eat the babies and soon after birth the mother will eat her own offspring too. Make sure you have gone to the pet shop to get the right breeding option for your fish.
The gravid spot, right above the anal fin, turns darker. Your guppy will have a box appearance, and her inside by the gravid spot go up in a 80 to 90 degree angle. She will breath fast and act weirder than normal. Her eating habits will change.