I've heard of them dipping tanks at farm sales, livestock auctions, or other farm related "events"
I've also seen people at my local station that sells on and off road diesel, that have been stupid enough to pump off road into the trucks tank on their trucks. That station is on the main road in town, and the state, county and local police stop in there all the time. I'm paranoid enough that I don't even like driving one of the diesels up there if I'm getting red diesel for something here at home. I know a guy that was extremely lucky. He was hauling campers and had a DOT number on his truck and burned red diesel all the time. Don't know how he never got dipped......