After any sort of collision
, it's always recommended that you take pictures at the scene. Pictures should be taken of both vehicles, the location of the crash, and the other driver's info and license plate. When your insurance company investigates the crash and assesses the damage on the car that you hit, they will likely be able to tell if there was damage on the vehicle prior to the accident. I would also recommend letting them know about the existing damage when you first give your statement so they can be aware of the details before they begin their investigation.