Open it up. If the insides turn too dark brown or black, that indicates that the flesh has spoiled. However, some people deliberately wait to develop the banana liquor that gives such depth and richness to the baked goods. After they get freckled, they will turn black and get a little shriveled. Place them into the fridge, and after a bit of time, they'll start to ooze their liquor which means the sugars are almost fermented. You can even smell a note of alcohol. Use in a banana bread recipe. I have yet to try this method.