On some species you can tell just before they go through partuition. They are pulled so taught that if you get the light just right you can see the young'uns. I know I've seen this on an H.arizonensis in a pet shop once.
I think flat rocks give birth over a period of several days (there was a recent discussion about this on the scorpionfans yahoo group), so you'll probably get more. Dave Gaban recomended seperating the babies when they left the mom's back and rearing them in individual containers as they were much more cannibalistic than most.