Most cookie recipes will give you a detailed description of how the cookies should look when they are finished baking, so it's important to read your recipe before you start -- even if you're an experienced baker. Always check cookies at the minimum baking time. While there is no test that works for every type of cookie, here are few common ways to tell whether the one you're making is finished baking.
How they should look: Firm, with a golden edge or bottom
Golden brown/light brown edges are used to determine doneness in many drop cookie recipes, such as chocolate chip cookies, oatmeal cookies, and some peanut butter cookies. Do not let the edges become dark brown or the cookies will be overbaked.
Other drop cookie recipes, such as those for sugar cookies, recommend baking until the cookie edges are firm. This means the cookie will look set up and no longer appear soft and melty. To determine how firm a cookie's edges are, you can gently nudge them with a spatula or lightly press the edges.
Another doneness guide on soft, pale cookies -- such as sandies, peanut butter blossoms, and sugar and spritz cookies -- is light brown bottoms. Since the color of the bottom of a cookie is not apparent at first glance, you need to gently lift the edge of one of the cookies from the cookie sheet with a spatula and peek at the bottom.
How it should look: Firm and set in the center, with brown bottoms
Although shortbread cookies are often done when the bottoms are brown, shortbread that is baked in rounds or wedges may break if you try to lift it to check the bottoms. These cookies are tender by nature and are fragile when warm. A good indication of doneness for these and other butter cookies is that the centers will become set and firm.
Chocolate Cookies, Gingerbread, and Other Dark Cookies
How they should look: Golden (or dark) brown, depending on the type of cookie, with firm edges
Brown edges and bottoms can't be seen on some dark cookies, such as gingerbread and chocolate cookies. Instead, these cookies are done when the edges become firm. You can gently nudge them with a spatula or gently press the edges to test firmness. A hint of brown may be noticeable on the edges.
Brownies and Bars
How they should look: Firm, with edges that pull away easily from the sides of the pan
Brownies are done when they start to pull away from the edges of the pan. Recipes for Cake Brownies or Buttermilk Brownies may recommend inserting a toothpick into the center of brownies to see if it comes out clean.
Doneness test: Completed baking time
Fudgy brownies don't have a specific doneness test because there's no easy way to check that they are done. In most cases these recipes give only one baking time and should be removed from the oven once the timer goes off.
How they should look: Topping should be light brown
Bar cookies with a crumb or streusel-type topping are done when the topping turns light brown. Even toppings made with brown sugar take on a golden color when the sugar caramelizes. Sometimes the bottom crust is partially baked before the filling and topping are added. Be sure to remove the crust from the oven as directed or the crust will burn when the bars are returned to the oven for the final baking.
Doneness test: Varies
Although many cookies share similar characteristics and indications of doneness, others show entirely different signs of doneness. For example, Fudge Ecstasies are chocolaty cookies that contain little flour, so the typical indicators for doneness won't work. These cookies are done when the edges are set and the tops have turned a dull color and crack. Always be sure to refer to the recipe to know how to test the cookies.