Personally, I have never seen a fuel check though I suppose they get done time to time.
Jack, that's an interesting point.
I remember going to El Mirage in the '90s, and I believe it was Chuck Kalbach's car being tested for fuel with an electronic insert that was put into the tank on a long stick. I asked him about it, and he explained that alcohol and nitro methane have different resistances, so that is how they were able to distinguish gasoline from fuel.
Maybe it's no longer possible to do that - gasoline formulations have changed dramatically in the last 20 years - but I can't ever say in the times I've been to Bonneville I ever saw gasoline tested in this manner.
Maybe I just missed it.
I know after my rookie pass, after I pumped out my tank, they beat on the bottom of it, it made a "clank", and they sealed off my tank after I filled up.
If they don't check after a record run, well, heck - A quart of nitro methane could easily be put in the tank through a vent.