Here's how to tell
There are two easy ways to tell if a hull is cored. First, the taper of the two skins from core to non-core is visibly apparent from the inside, at the hull-deck joint and other places where the core must end. Second, look for the light. From inside the boat, look for light shining through the hull. It may help to have a friend outside with a strong flashlight. Fiberglass is translucent, and thin layers of gelcoat and paint are only partly effective at keeping the light out. Coring is opaque. Again, the acid test is when you find the edge of the core. If the entire hull seems opaque, you aren't running the experiment right.