See here, on page 4...
http://www.eaa.org/education/fuel/autogas_vs_avgas.pdf

Basics, are a tall and relatively thin cylinder, holding perhaps a cup
100ml) of fluid... Make a mark at roughly 10% of the way from the bottom...
Make another mark near the top... Fill with water to lower mark,
accurately... Fill with gas to upper mark, accurately... Put hand now
smelly over the mouth and shake it well... Let stand to settle the water
for a couple of minutes whilst you do other things..
If the water level to the lower mark has now moved upwards you have (had)
alcohol... If the gas level at the upper mark has moved, you goofed - or
the cat has a bad huffing habit......

If nothing has changed, carefully decant pour the good gas back into the
can and throw the water away...
denny

"Jay Honeck" wrote We haven't been testing our
auto gas for alcohol, but I would like to.