How can a rental place tell if you smoked in the car?

You can smell it.

I work as a car prep for a rental car agency, and I can tell in an instant if a car has been smoked in. People who smoke don't realize it, but the odor is extremely obvious. Air fresheners don't remove it.

To really remove a smoke smell from a car, you need carpet deodorizer and/or some kind of odor neutralizing spray (like Febreze). Also, you need to wipe down all the non-fabric interior surfaces, especially the inside of the windshield.

Sometimes there are more obvious signs. It's typical for me to also find that the door armrest pockets have been used as an ashtray. The smell from that does not come out, even if the pockets are cleaned. Loose ash inside the car is also common.

It is also worth pointing out that yes, marijuana DOES count as smoking in a rental car, because the results are the same. A strong odor, ash (often also loose weed bits), and potential damage to the car (burns on the upholstery).

I don't know about the policies at other companies, but no, we will not report you to the police if we think you smoked pot in a car. Now, if you leave your weed behind, that might be another issue. But just the smell and a bit of flakes? Nah. We don't care. And it's much more common than you might imagine.