Well, you'll need to manhandle them. Secure the wings well, because they can hurt if they get you with a flapping wing. Flip them onto their backs with their back on your lap and their head sticking out between your arm and torso. Gently open the vent. Try not to push down, you don't want to be pushing down on them, that can injure them. Instead, gently pull it open. If you have a gander, his penis will be exposed.
This video shows you how to do it.
Geese are seasonal layers. Females start to lay for the first time the spring after they're born. Then after the breeding season, they stop again until the next year.