It's not a criminal investigation, though it seems strange they wouldn't give you the courtesy of asking first. They have the right,and if you refuse I believe they can impound the vehicle until they get a warrant.

Odd that the State Patrol was doing it...I thought it was Dept of Revenue guys...was it a blue Tahoe or Suburban that said Motor Vehicle Enforcement on the side? That would be DOT, not State Patrol. "Here" I know guys who have been stopped by the side of the road for DOT checks but never had fuel sampled.

I've been checked a couple times, but I don't remember what the patch on the guys shoulder said. It's like Rich said, stay legal and it won't matter if they check or not.