Jet A you smell and put a drop on a white piece of paper. Avgas
evaporates, Jet A leaves a ring.

Jeremy Lew wrote:
Is this the kind of thing which can be caught by looking at the color of the
fuel in the fuel tester? I don't know the color of Jet A, but I was taught
to look for the blue color and that if two kinds of fuel are mixed it all
turns gray. Anyone know if in practice this would be easily detectable in
the fuel strainer?