You can tell if the concrete is sealed by checking for a wet or oily look on the surface. If the concrete has a wet or oily look, it has not been sealed and needs to be sealed. Also, you can check the surface with a piece of dry paper towel. If the paper towel sticks to or is hard to remove from the concrete, it has not been sealed and needs to be sealed. Also, try to drop a few water droplets on the surface. If the water droplets bead up and stay on top of the concrete, it has not been sealed and needs to be sealed.