I'd think about drying the boat well, putting it up on sawhorses, and carefully pouring a quart of two of water into it. If you don't see a leak immediately by observing the outside, try tipping the boat so the water runs first to the bow and then to the stern. Another possible point of leakage could be the sheer seam, so put the boat up on edge and see if anything leaks out. Once you find the leak it shouldn't be that big of a job to patch it up.