I have witnessed fuel compliancy checks and they have nothing at all to do with the D.O.T.
The Department of Revenue is who investigates improper fuel usage.
They don't tell you anything at the time of inspection.
What is done :
A road-block is set up, usually on a bridge.
Not all diesel vehicles are stopped, but a representative portion; maybe every third blue truck, or every fifth white one.
Your driver's license number, V.I.N. number, and license tag number are recorded.
A siphon tube is inserted into each tank and a sample of fuel drawn.
This sample is placed in a clear plastic sample-bag, much like those used in the milk-hauling industry.
A number tag from the paperwork that has your information is affixed to these sample-bags.
The samples are taken to a lab and analyzed.
You are told to drive safely and sent on your merry way.
Months later, either you hear nothing at all, or someone knocks at the door with a warrant.
At least that is how it is done around my area.