They will brown around the edges and begin to pull away from the sides of the pan. That's the best indicator. Also, when I touch the center of the pan of brown sugar brownies, they feel firm, but still have some give (movement) indicating the softness inside. When you feel that slight movement you'll be tempted to bake them a little longer. But, please don't. They will continue to "bake" a little outside the oven, just from residual heat. And, they will set up as they cool. Over-baking will result in dry and disappointing brownies. We aren't trying to achieve the dry, cake-like texture of traditional blondies. Brown Sugar Brownies really are their own unique style of cookie bar.