How to tell when cheesecake is done? I'm baking a cake that shall be categorized as "light cheesecake". The recipe says when the top of the cake is firm, the cake is done. So I baked for an hour, opened the oven and found the top of the cake baked right, but when I cut up the cake the inside was too wet. So I had to bake for another 20min to get the inner texture right, but the top is dry already(and stiff after cooling). What have I done wrong?

My recipe: 33g flour, 75g yoghurt, 140g cream cheese, 50g whipping cream, 50g sugar, 2 eggs for an 6-inch pan. Baked in water bath, oven at 160 Celsius.
Cheesecake is a custard, so your very best indicator of doneness is temperature.  There are a range of opinions, from about 150 F to 165 F / 65 C to 73 C, measured at the center of the pan, in the center of the layer.

If you don't have an instant read thermometer, generally, you want a thin knife to come out clean from the center; similarly, when the pan is shaken a bit, only the center should jiggle slightly.  These indicators are not as good as temperature, so you will need to gain experiene to interpret them.

Once you are measuring properly, problems with the top of the cheesecake being done, but the inside being too unset may be mitigated by:


Baking in a water bath (which you indicate you are already doing)
Baking at a lower temperature (within reason; you indicate you are already baking at a fairly low temperature)
Baking in a wider pan, so the cheesecake is more shallow
Do not use convection


Since you area already using a bath and a low baking temperature, if you are still having problems once you start checking for doneness by temperature, I would suggest checking your oven's calibration with an oven thermometer to be sure it is not running hot.