It isn't just the Feds that test diesel fuel. State tax agencies may also test.
They will/can test ANY diesel fueled vehicle if they choose to do so. If they find traces of red dye, they can and will assess the penalties.
They can do the same for ANY "special fuel" vehicle. If your vehicle runs on propane or CNG, you MUST pay the highway fuel taxes on all the fuel you use. If you convert your diesel to run on home-blended bio fuel (used fryer oil), you MUST pay the highway fuel taxes on every gallon you make! If you work at an airport, and have access to waste jet fuel, and burn it in your on-highway diesel vehicle, you MUST pay the highway fuel taxes on every gallon you use!
Will they catch you if you don't pay the taxes? Maybe not. How lucky do you feel?