In the past I've often thought I had fuel contamination as
"verified" by smelling the oil. Wrong! Every time my nose
was wrong, with the UOA proving it. I no longer trust the
"sniff" test (at least with my "sniffer").
If cost of doing a UOA is a factor, your local NAPA store sells
a test kit for $14, part number 4077. This is will tell you
if you really do have fuel contamination.
I am unaware of any home chemistry experiment which will
directly tell if you have fuel in your oil. However, it might
be possible to infer indirectly. If you have a lot of fuel in
your oil, it will affect viscosity. You could put a sample of
virgin oil in the freezer along with a sample of used oil. After
they are well chilled, see if there is a difference in their
ability to pour. If the used sample pours faster than the
virgin sample, it could indicate excess fuel in your oil.
The cause of fuel contamination can be leaking injectors,
and misfires. Defective or misadjusted carburetors are also
a problem. Does your car misfire a lot? The misfires could
be causing fuel contamination.
One way to test for leaking injectors is to attach a fuel
pressure gauge to the fuel rail. There is usually a Schrader
valve on the rail which allows you to easily do this. Check
the fuel pressure with the engine running. Look for excess
pressure. Make sure the vacuum line attached to the pressure
regulator is not cracked or missing. If you have excess pressure
it could cause injector leaks. Next, shut down the engine. Leave
the pressure gauge attached.
The pressure should hold for a considerable length of time. If
you come back an hour later and the fuel pressure has dropped
considerably, then you might have leaking injectors. Some
drop in pressure is normal. It depends on the car. A drop of
10 or 15 percent after an hour probably would not be a cause
for concern. Check you shop manual for more information.