You'll know it when you see it. And don't get overly excited. New shrimp moms often drop their first clutch of eggs before they hatch. My first female had at least 30 eggs and out of that I think I got five babies.
The shrimp females learn to hold onto their eggs after the first or second batch, and do a much better job as they gain experience.
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If you feed them more proteins and keep the tank warmer, they will grow faster and have to moult more. This may increase the rate of breeding, but it also lowers their lifespan. You also have to be more vigilant with tank parameters, otherwise you end up with a bunch of half moulted carcasses...