I hold the plywood over my head in Home Depot to check for voids. There is usually enough light in the store to sort out bad sheets. After stitching a hull together I check again using a 100 watt light. If I see a void, I use my Dremel with a grinding type bit (pick one that looks like it will do the job) to cut through one skin. I then fill the void with thickened eopxy, sand smooth when cured, and fiberglass over that. So far, no problems; but then, I treat my boats gently --- no rockgardens!
Hope this helps.
Dave