Best way I can figure is to put some in a jar and mix it with water. Ethonol is put in gasoline and will mix together with water to separate out. However, it'll likely have the same effect if there's any biodiesel.

As for any pure petroleum bases products (pure gasoline, mineral oil, whatever), you'd likely have to get a bit more scientific with burn rates and ignition temperatures.

Yes. Burning it tells you. Also requires a flame or extreme pressure to ignite. Spark won't do it. Only gas has a low temp vapor pressure.

yes, because of all the anonymous donations and the expensive equipment that is the livlyhood of the owners, they are testing all the fuel. It is quite organized. The stuff that was 'returned' to them by the police was mixed with 50% water and would have caused hydrolock in any vehicles which tried to use it.