Welcome aboard:
Start by checking a couple of obvious things outside the boat: look at the rub rail and see if there's a spot that looks like it might be leaking, and check for cracks in the hull that might be leaking. Check all of the thru-hull fittings too (drains, vents, cleats, etc.)
While it's dry and on the trailer, take a flashlight and look for traces of water and see if you can follow it back to a source (or something that might be a source). Use a cellphone with a light to take pictures where you can't get your head back into.
If nothing comes of that, you may have to take it out and watch where it fills from.