The majors absolutely can tell if it is their additive package in a stations tanks, and they can, and do, sample branded stations to make sure the right product is in the tanks.
Enough bad samples to show a pattern of the vendor buying cheap gas and passing it off as branded will get the brand pulled.
Lawsuits are fruitless to stop the brand being pulled at that point.
I sell cheap generic unbranded gas at one of my rental properties, but I wouldn't put the gas I sell in my cars, or even my lawnmower. Only Shell goes in my gas burners.
Gas is not gas so far as I am concerned, but I like the idea of so many people thinking that it is.