You can tell that your cheesecake is done when it looks properly set and no longer jiggles. It is okay for only a small circle in the middle to jiggle, as this will set and firm up as the cake cools. Do not use a knife or sharp object to check for doneness, as this will create a crack on the top. It may also, give the wrong impression of doneness for cake recipes using a large amount of sour cream.