I just came across this and though it might be helpful now that we are running in to the holiday baking season. This is from http://www.cooksillustrated.com:
We think most cookies are best when they are chewy. This means taking them out of the oven when they are slightly underdone—in fact, the cookies are often so soft they will droop over the end of a spatula. Bake cookies on parchment and cool on baking sheet for a few minutes; after they have set up slightly, slide the parchment onto a cooling rack. if the cookies have crevices, the crevices should appear moist. When baking smooth cookies, look at the edges, which should be lightly browned; the center should be set but not fully dry.
I also find that when you can smell them, time to check the oven as they are done or just about done.
Good Morning Gloucester 