You can test this by bumping the pan a little; it should only jiggle slightly. In my oven, I find that the point at which the filling is set is also when it starts to get very light brown on top. Let cool on rack, unmold tart pan and serve. I actually prefer this tart completely chilled, which makes it a great dessert to make in advance of a dinner or party. Meyer lemons might seem the obvious choice here, and they will work, but I actually prefer this with the sharp, less floral, intensity of standard grocery store lemons (Eureka variety).