So, what follows is quite possibly the best thing I got out of my physics
degree.  When you wrap the fingers of your right hand around a wire with the
thumb extended (like in a thumbs up gesture) and the thumb pointing in the
direction of the electrical current in the wire, your fingers show the
direction of the magnetic field that is induced around the wire.

Which was valuable to me for some brief period of time when I had to draw my
vectors in the right direction on an exam.

However, it also works for (right-handed) screws, nuts, bolts, etc.

So if you're under the van* reaching up to a bolt that's pointing down
toward you and you have no idea which way is which, just wrap your right
hand around the bolt with your thumb pointing the direction you want the
bolt to move.  Your fingers will point in the direction that you should turn
the bolt in order to make it go that way.

Or, if you've got the wrench on and you're tugging and having a moment of
doubt, you can curl your fingers (right hand only!) around the bolt in the
direction you are turning your wrench, and your thumb will tell you which
way the bolt will move once the penetrating oil finally kicks in.

mike

* vanagon content!  (But seriously, this came up because someone mentioned
it in the discussion about getting the bolt off that holds the spare in.)