Or more accurately, as Knott has pointed out, when you sit on a flat hard bench (wooden ones are more likely to be carved into a curved shape than stone) with your legs, spine and pelvis arranged to approximate your riding position, do you feel pressure in your soft parts?

If the test suggests you do need a cut-out, there's the Selle An-Atomica (search on that, many threads here as well).

Sort of like the pencil test Zen reminded us of the other day, but at the other end.