Pour some in a glass and let it sit a day or so. Anything "not" gas will separate. Ethanol is lighter than gas and ends up at the top. You can really smell it when it separates. Based on what I've seen in a glass like that, I'd estimate probably 2 or 3% in the supposed "non" ethanol gas around here. I know it's not scientific, but that's the only way "I" can do it.