Josef, I have some limited knowledge about gasoline since I was in retail gas for over 20 years but no longer, recently. Anyways you already have the test kit at home. Water and a glass jar with a lid. For example put 1” of water in a glass jar and mark it with a marker. Fill a good amount of gasoline, lets say a cup or 2. Put on lid and shake it well. Let it settle, overnight for example. The water and ethanol will combine, water is heavier than gas. So now win it settles you should see the water and ethanol will now be taller than 1” and above your marked line. The ethanol is only 10% here in this state so that could very per state. So 10% of a cup isnt very much, so using a little more gas, just to see a good separation.
Also ehtanol is 113 octane, so when it does seperate, you are basically running racing fuel thru an air cooled engine, not good. Hey Im all for using 10% less oil but ehtanol is more expensive than gas, it’s subsidized from the government. I dont know who is really winning here but now its just common thing. Lastly when we use to splash blend, buy gas and ethanol seperately, the ethanol would jump the octane 2 points. For example we would buy sub par 85 octane and 10% ethanol to make it 87. So if you take out the ethanol, you would be losing about 2 points of octane. Using a higher grade first, then you may not need an octane booster to get an 87 final octane for most engines. Most states very per signage and ethanol standards.