If you mean checking her vent to see if she is laying, I find it's easier if I start by checking some that I know are not laying (roosters, broodies, anyone too young to lay), and then ones I do know are laying, and then the one that I am not sure about. That gives me a good reminder of what signs I am looking for (vent) and feeling for (bones below the vent.) A hen that just recently stopped laying, or is getting ready to start, can have confusing in-between traits, but I can usually be sure about most of a flock, with just a few maybes.
Given how fast it is to check butts (especially if you grab the birds off the roost at night, one after another), I prefer to do that before I bother caging any bird individually-- because if she is not laying, I can save several days of having her in a separate pen. A not-laying bird is usually the easiest for me to identify.
I agree that is a surefire way to get her eggs, if she is laying.