Buy a Briggs and Stratton 100023 Test Kit. (a graduated glass container with cap that is 6" tall X 3/4" Dia.) cost about $6.00. Will test from 0 to 30% alcohol in 1% increments.
Instructions:
Add water to the water line.
Add gasoline to the gasoline line.
Replace cap and shake vigorously.
Stand vial vertical with cap upright. Let stand for 10 minutes.
Locate the gasoline/water seperation line and read scale.
The seperation line will indicate the percentage of alcohol if present.

 

Ummm.... Yes... sort of... that is over simplified but will get you in the ballpark if you only want/need an approximate value.

By the way, there is no reason to have a 0-30% limit.
You can use this separation method to test for any percentage.

Here's the math to be more precise, but of course, the precision will be limited by the quality of your measureing device.

Cheers,
-John

My own tests and results: