There are two groups who can take a sample from your tank, any time, anywhere: the IRS and EPA. The IRS for not paying tax and the EPA for high sulfur.
The laws that created dyed fuel provide the basis for the search--dyed fuel is available, and by owning a diesel, there is a possiblility that you have put dyed diesel in it. That's all the reason they need. No different than asking for a driver's license, registration, or proof of insurance.
The laws that created dyed fuel provide the basis for the search--dyed fuel is available, and by owning a diesel, there is a possiblility that you have put dyed diesel in it. That's all the reason they need. No different than asking for a driver's license, registration, or proof of insurance.