When they stop eating, when they shed, etc., you cannot determine when a snake will give birth based on those things. Snakes will do those things even when they are not gravid.

If you see them mating, then you can guess 70-100 days later, the babies will be born. If you don't know when they mated, then it is difficult to know when they are going to give birth. You have to know the date of the mating, and even then, you can't know the exact day, or even what week the babies will come. Gestation period varies.