There are fancy marked test kits with tubes that will read a % when the fuel is shaken with water, as well as simple quick dyes that will either mix and turn the fuel a colour if there is any ethanol present (or blob at the bottom if none).
But yes, the quick and dirty method I like is the shake test in a jar or the like. Water bottles are plentiful these days and already have ribs.
For a slightly more accurate test, if you have a 100ml marked cylinder....you would add 10ml water and 90ml fuel, shake, let stand 5-10 minutes, note the level of the separation layer and subtract 10 to get your %.
Below is a c/p from aviation website...
To determine if ethanol is in the gas:
-On a test tube or olive bottle six or seven inches long, make a permanent line about two inches from the bottom.
-Fill with water to this line, then fill the tube to the top with gasoline.
-Cover the tube, agitate it, and let it stand.
-The ethanol and water will mix and separate out together. If the water level appears to have increased, the fuel contains ethanol and should not be used. Ethanol percentages of less than 5% can sometimes give a reading below the line. Therefore, any deviation in the water line indicates the presence of ethanol and should serve as a basis for rejecting the fuel.
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