Commercial vehicle safety inspection. They check all lights, look at tires, occasionaly they will check for slack in air brake adjusters, they will sometimes check for proper operation of low brake air wanring buzzer, check for fire ext, flares/triangles, daily vehicle inspection form, drivers logs, proper placards, proper manifests for haz mats, tire damage...etc...etc...etc..
They even weighed one of our unloaded vacuum tankers on the side of the road one day, to make sure the driver wasn't running an overweight vehicle.