Some cars have OEM flex fuel sensors and it maybe able to read the value with OBD2 custom PID.
Beside innovate motorsports, Haltech ECUs also have an optional flex fuel sensor, as may a lot of aftermarket full ECUs.
This sounds like the most plausible answer although it doesn't make sense to me why someone with a flex fuel vehicle would want to pay more for E0. I don't know the guy, we were both customers waiting inline at the mower shop passing the time, I have never seen the guy before so I doubt I'd ever run into him again unless by chance.
The reason I was trying to verify his friend being able to tell how much corn is in his tank is my concern for Casey's not selling a true E0. Three months ago Shell started selling E0 again so I started using it. I've been thru about 3 tanks now and noticed I'm getting about 25ish more miles per tank (all city miles) compared to Casey's E0. At first I thought, could it be the TT fuel but after hearing this guy say Casey's E0 isn't 100% straight gas makes more sense to me for the increase. My Buick runs just fine on E10 but I get a 2-3 mpg hit when using it.
I also realize some might say it could be winter to summer gas but to be honest the only time I see a significant difference between the two is when I let the car warm up before driving it, other wise I can't see a difference in mpg that is noticeable.
I'll have to search for testing E0 if I have to go back to Casey's.