The best way to tell if your chocolate mug lava cake is done is to check the center. When it's done, the edges will be set and the center will be gooey and runny. To check, insert a toothpick into the center of the cake. If it comes out with wet batter, it needs more time. If it comes out with just a few crumbs, it's ready to eat. Keep in mind that microwave wattage can vary, so adjust the cooking time accordingly.