You can tell your potstickers are done in several ways. The best way to tell is that the water has evaporated from the pan and the dough is beginning to crisp up slightly.  You should also pay attention to the color of the filling. You may not be able to see much through the dough, but your ingredients on the inside should darken just a bit. The dough on the exterior will become just a tiny bit more translucent as it cooks and then become golden brown as it sticks to the pan.