Once oil is mixed it’s too late to do this but in cases where someone has had a potential bad gas complaint what I’ve done is take a sample, dump a few tablespoons on concrete away from everything, and throw a match on it.
Good gas will go up instantly. Dubious gas will ignite slowly. In one instance a burning match did not even ignite whatever it was that came out of the fuel tank of the car that was towed in shortly after filling up. No water or solid particulates at all; just an odd liquid that refused to burn at all.