At the check-in you'll go through the rental agreement with the car owner. You'll need to check the position of the fuel gauge and mark it on the agreement. When returning the vehicle, you'll just need make sure to return it with the same amount of fuel as you received it.
The price of the rental does not include fuel consumption, and our policy is to refill as much as you used. Usually it is easier to check the gauge with a full tank. It is possible for owners and renters to agree on a compensation amount if the tank is not refilled.