The prepaid fuel charge should appear on the contract when you pick up the car, along with the "fuel out" level. If the contract indicates half a tank, you should only be charged for half a tank. However, sometimes the "fuel out" level printed on the contract will be in error, so when you step into your vehicle, compare the fuel-gauge reading with what is printed on the contract. If there is a discrepancy, immediately snap a picture showing the fuel level and odometer reading, and go back to the rental counter to have your contract reprinted.