Rental car companies will inspect cars carefully upon return; do yourself a favor and perform your own inspection before driving off the lot. A little extra time and effort at the beginning of your trip will save you undue charges later on.
Walk all the way around your rental car and inspect the exterior. Take note of even small dents and scratches. If possible, check the roof. Inside the car, look for rips or tears in the upholstery and fabric. Check the ceiling and back seats. Scratches, dents, and damages to glass or bumpers are the most common claims.
Document any damage in writing on an official form or rental agreement from the rental company. Be sure to get an employee to verify and sign the forms. Both the rental company and you should have a copy of any forms that indicate pre-existing damage. It is very important that you have something in writing signed by an employee. In a perfect world, an employee would accompany you to your rental car and assist you with the inspection. In reality, you may have to track someone down.