How to Test for Ethanol in Gasoline
This simple test for ethanol content is fairly easy to do at home. First, siphon out a small amount of your gas. Then, pour some of it into a measuring cup. If you can find a graduated cylinder that holds 100 mL at an educational or science supply store, that would be ideal, as it will make calculating percentages easier. If you have the graduated cylinder, pour gasoline up to the 50-mL mark.
Now, add 50 mL of water to the cylinder. Put your finger over the top of the cylinder and shake it vigorously, then set it down for twenty minutes. As the sample sits, the alcohol will come out of the fuel and mix in with the water, and you will see a clear line where the alcohol/water mixture separates from the gasoline.
The gasoline will rise to the top, because it is lighter than water and alcohol. Remember that, initially, you had 50% gas and 50% water, with an unknown quantity of alcohol/ethanol in the gas. Now, let's say that the line separating water and alcohol from gasoline is at the 80-mL mark. That means that 30 mL of the 50 mL that you had thought was gasoline was actually ethanol. That means that you bought 60% ethanol fuel for your car -- which will be deadly for your engine.