If you look at the surface of the belt on the smooth side there will be fine cracks in the surface. Very bad ones will have missing pieces in the ridges. ===ANOTHER ANSWER=== ANOTHER WAY to tell is by the mileage. If you have 60,000 miles or more, I would chage it or have it changed. It is well worth the investment, because if the belt has been slipping and gotten glazed over, or just regular wear and tear, you don't want to wait until you are on the road, broken down saying "damn I should have had that stupid belt changed!! A new belt will never hurt your car and it could save a LOT of aggravation! Hope this helps. P.S. I'm not sure what the first poster meant by "looking at the surface of the belt", BUT the top surface that you can see at a glance will always look "good". You have to twist the belt to see the side that has contact with the pulleys to see wear and cracks.