re: Is there a way to test ethanol-free gas to make sure it is really ethanol-free?

Yes, in a Petroleum Laboratory setting there's several options, Gas Chromatography being the most reliable which produces quantitative data. There are other testing options which are only considered "indicators" and do not provide an actual numerical value.

If you were to use the Gas Chromatography option, ASTM D5599 Or D4815 will measure the Ethanol in Vol%. For example, 10% Ethanol in Gasoline would be determined by those test methods mentioned above.

If you're concerned about trace amounts, ASTM D7423, measures Alcohols (In this case Ethanol) in PPM levels (Parts Per Million). So, a Gasoline containing 10% Ethanol ˜ 100,000 PPM. So, 1 PPM of Ethanol ˜ 0.0001 vol%.

I'm sure there's home test available, but they would only be considered "indicators" not expressed as a number.

Hope this helps.