Re: Fuel in oil - where to start?
Gravity, yes. If you're going to change the oil anyway, do a simple leakdown test, pull the plugs, fill with kerosene or gasoline, cover (to prevent evaporation) and come back tomorrow. Add a few drops of UV dye if you really want to double-check. Or, just borrow/rent one of the newfangled leakdown test kits that uses air pressure.
Certainly cheaper to check, than to tear into things.