The way to approach this is to get an ultrasound machine at the shop, charge 150.00.

Have the customer sign a document stating "this shop is capable of finding most types of damage to carbon fiber frames but they cannot guarantee that there is no "hidden" damage to an inspected frame. They can determine certain types of frame damage and upon detecting that type of damage will let the customer know that a frame is unsafe to ride. The shop cannot be held liable for damage to a frame that is undetected."

Have a lawyer do the word smithing.