Cracks usually occur near welded areas, or where the frame is butted. Probably the most common spot is the underside of the down tube, just below the head tube. On carbon frames, it can be difficult to tell if you’re looking at a scratch in the clearcoat or a crack in the frame. If your fingernail can catch on the blemish, it might be a crack. If you have your suspicions, go to the shop.