Two ways that I know of. One, go into a closet, turn out the lights, and shine a flashlight into the waders. You should see a pin hole of light wherever there is a leak. Mark it with a felt tip marker so you can find it to fix it. The other method, which doesn't seem to work as well, is the bathtub full of water and dunk them looking for air bubbles. I think some leaks require more time and you don't always see an air bubble where there is a pin hole.
Good luck. Jim
