How do you report a bad truck driver?

Get the truck's license plate if you can, see if there is a carrier/company name on the truck. Make note of the time of day, which roadway, and if it's safe to do so, see if you can spot a "fleet number". The more info you can safely get, the easier it will be for the company to figure out exactly who was driving. Call the carrier with this info (you can look them up online or in the phonebook), or make a call to local law enforcement.