This can be a little tricky, because if your frittata is not cooked enough when you put it under the broiler, you can brown the top and still have some of the filling not cooked through in the middle. So it may take a little practice until you know exactly what you’re looking for. The best tip I can give you is to make sure the center of the frittata doesn’t jiggle when you take it out of the oven. If the top is well browned but the frittata is still jiggly, just turn off the broiler and let it sit in the oven for as long as you need for the center to not be jiggly. The frittata will continue to cook with the residual heat from the oven.