I really don't think that putting gasoline in a diesel engine would be covered by a lemon law. It isn't the fault of the manufacturer if a person put the wrong fuel in it.
Years and years ago, I bought a new car and traded in a diesel VW Jetta for it. The next day, the VW dealer called me to tell me my car had been towed to them and they wanted to know what was wrong. When I traded it in, it was at about 1/4 tank of fuel and I told the dealership that I had traded it in and my guess was that they filled it with gasoline. His reaction: "I really hope they didn't do that..."
With all that said, it should be obvious once they take the fuel out of the tank. Gasoline and diesel fuel look and smell very differently. I'm sure they'll chemically test it just to be 100% sure, but I think I would be able to tell immediately just by the color or smell.