It depends on the fish, what signs they are going to show. Common signs include:
Eating more/less
Hanging out in one spot
Being aggressive with other fish
Chasing other fish away from her hiding spot
Sitting on the bottom
You seeing eyes very clearly in her gravid spot
Swimming in one place
That's some of the common ones, but every fish is different. Some may exhibit a lot of these signs, and some may exhibit none. Once she's given birth a few times, you'll know what she does beforehand, and you can be prepared for the next time.