It takes practice. I still bite at it.
I'm usually fairly accurate (I'll count say 4 maybe 5 and end up with 5 eggs or 8 possibly 9 and get 9 eggs) but it took practice and searching for info on how to do it.
If you let them relax and slide back in the tub while you palpate, it makes it a bit easier.
Like John said, check RDR's site and there was also a youtube video on the net at one time showing how to palpate.