Answer: The most reliable method is also the easiest method. Run the engine until it is fully warmed up, shut it off, and then grab hold of the dipstick. If it is too hot to handle, your engine is running too hot. If you can just hold on to it, even while switching from one hand to the other, your engine is not overheating. Yes, it's a rough method and a bit subjective, but is reliable enough to diagnose a too-hot aircooled vw bus or bug engine.