once you've had them a while, you'll learn their behaviors and pick up clues from it that will give you an idea of who is gravid and who is not. They are individuals, but you will learn that certain females stop eating, others eat more, some soak frequently, others bury and wait, others bask in sun and are more active. Some act as if they are marching around the world and won't stop moving. Some very close to laying begin sniffing the ground a lot and doing test holes.
Other than that, keeping good records of their weights will help.
Also, when they are close to laying you can palpate them. This is a method of feeling inside the body cavity for the presence of eggs.
You must be extremely gentle doing this. Otherwise you may injure the turtle.
Grip the back legs in your middle finger and thumb. Use your pointer fingers to feel the soft skin inside the shell in FRONT of the back legs ( holding the turtle so it faces away from you ). Calcified eggs (eggs which have formed their sturdy shell) will feel as rounded bumps. It takes practice to get used to this, but after enough experience you can even count the eggs to within a close margin or make good guesses at laying times and clutch sizes.