To make sure your muffins are fully baked, insert a toothpick or a cake tester into the center of a muffin. If it comes out clean or with only a few crumbs attached, the muffins are ready. If the toothpick comes out with wet batter on it, the muffins need more time in the oven. It's important to also check that the muffins are evenly baked and not undercooked in the center or overcooked on the edges.