Just put a few ounces of unoiled fuel in a glass jar. Then put a similar amount of suspect oil in a similar jar.
The oiled fuel will probably look slightly orange / brown in colour.
Standing both glasses in sunshine will probably enhance the coloration due the effect of UV on the oil which darkens it.
Works on the oil I use, Turbonycoil and probably similar other oils used in full size jets, BP Exon, Mobil.