Normally if an insurance claim is filed the adjuster handling the claim requests copies of the previous rental contracts. Most of the time, if the damages were there prior to you getting the car it will be noted on at least one or two of the recent contracts. If not, please accept the possibility it happened while you had the car and pay the money you owe. A crack in the windshield is not really minor damage, and you can't expect people to just take care of the cost simply because you don't want to pay it.