A good hire company walks round the car with you. Here is what to look at, and why.
Walk round it
Look at every panel and both bumpers, and ask the desk to note any marks on the form. Take photographs of anything you see, with the date.
Check the tyres and the spare
Look at the tread and the pressure, and make sure the spare, the jack and the wheel brace are all there.
Inside
Check the fuel level matches the form, the air conditioning works and the warning lights go out after starting.
Know the rules
Ask the fuel policy, the kilometres included, who may drive, and the number to call if something goes wrong.