Does anyone know of a sensor for how much alcohol (ethanol) is blended into a particular gasoline sample? Or is there a method that can be done electronically by density or what?

In two-stroke ultralight aircraft engines, a lot of the engine troubles with corrosion, seals, and fuel storage are blamed on alcohol in the fuel, but pumps do not state the alcohol content and it's a tough and inaccurate procedure to measure. The only procedure I know is to put it into a jar, add a measured amount of water, and shake it up then let it settle. Alcohol absorbs water, gasoline does not, but I am unsure if this procedure is even a valid measure.

I have looked on the internet and saw several references to the 85% ethanol E85 vehicle having a sensor to determine the alcohol content in the fuel and adjust the engine accordingly, but others say the exhaust oxygen sensor was the source for adjustments. I didn't find any more information on what the fuel sensor was.