Hang them up and fill them with water. Then get a spray bottle with dishsoap and water in it and start sprayin. It will bubble a little when you find a hole.
Sounds dumb, but we've done it a ton of times and it works. This is the winter time method.
Since its summer. Put on a pair of shorts and put your waders on and wade out into a pond and stand there for a few minutes. If its going to leak, you'll feel it. Also you can turn your waders inside out and find the wet spots.