If you are still at the rental office, insist on a joint inspection of the car. If you took pictures of the car upon its collection, ask the rental employee to compare the current vehicle’s condition with them. If it turns out that the damage was already present before, have the check-in document confirm that no new damage has occurred during your rental.
If you received the charge after the rental when you are already at home, ask the car rental company to prove that the indicated damage was caused during your rental period. Additionally, ask for a damage invoice. In case you took pictures of the car upon its collection and return, have them ready to justify your point.