Put about an inch of water at the bottom of a small glass jar. Mark the water level with a sharpie. Put gas in the same jar, it will float on top of the water. If the gas contains ethanol, the ethanol will separate from the gas and mix with the water, making the water level appear to have risen. If the water level stays the same, ethanol free.
There are commercially made test kits which do the same thing but the amounts of gas and water are measured, so you can tell the ethanol percentage based on how much it has risen. I only cared to test for the presence of ethanol so I didn't measure the amounts.