The first test is to lightly prod the top of the muffins - if it springs back, chances are high the muffins are done. You can also stick a toothpick near the center of a muffin. It should come out mostly clean - a few baked crumbs are fine, but there mustn't be any raw batter. Finally, my preferred way is to use a food thermometer. I bake muffins to an internal temperature of around 210°F.