An ounce of prevention is worth a pound of cure: These days, I always take a picture of the odometer reading and fuel level when I return a rental car.

And it's always a good idea to check both at the start of a rental, to make sure that they agree with what's on the contract. Even on an unlimited-mileage rate, you don't want to return a car after one day when you've only driven 90 miles, and have the rental agency think that you drove 900 miles in one day (due to someone's dyslexia or fat finger when the rental before yours was closed out).