I've gotten in the habit of taking a photo of the dashboard after starting the car, showing the mileage and fuel level, then taking the same photo on the return. Also, a photo of the back of the car showing the make/model and license plate. Recently, some of my rental car keys haven't shown the license number, just a code, which presumably Hertz can connect to the car. It's not unusual for me to forget what make or even color of the car I'm currently renting.