Cody S. explained this quite well. I have a Chevy K2500 pick-up rated at 8600, if I tow a single axel 5x10 rated at 2950, I'm over 10,001 combined and by the book need a USDOT to drive around and cut 10 lawns. It sounds foolish but true. Most including myself are not in compliance w/ this.
Also the website he cut and pasted has a step by step walk through process to determine if you need a USDOT number displayed on your truck, and which state it would be a requirement in.
Only "DOT" police or law enforcent certified to enforce FMCSA Level 3-Level 1 could enforce the USDOT requirements. Most local, county, state patrol law enforcement would not even pick up on it as they could not enforce it and would most likely not even know or care about the requirement. If they did they would have to call someone that is FMCSA certified, usually a Trooper assigned to a CMV inspection unit, to conduct a "inspection". If the trailer is rated for 5000lbs or more it needs to be inspected, if used to make money, trailers over 7500lbs need to be inspected personal or business use.