Alcohol tester
I use a glass tube, 5mm diameter, melted to seal one end. A brush bristle is araldited about 25mm from the bottom. The tube is about 200mm long. I use a thinner glass tube to fill it to the mark with water. I then fill it up with petrol. My finger seals the end. I invert several times. The alcohol from ALL the petrol should go into the water. (Well, some of the alcohol from all of the petrol.) I've only once got a small rise in water level. I repeated the test several times, then put the petrol in my car.
At least I can honestly face the AAIB and Insurance company if I survive an engine failure.