Simple: To figure out which way something is threaded, figure
out why it's threaded that way. The reason for thread directions
on rotating objects with lateral load In general, the thread
direction is NOT chosen so that friction from the rotating shaft
will tend to tighten things. If that were the case, pedals would
be left-hand threaded on the right side and vice-versa. Rather,
it has to do with precession. If you hold a pencil loosely in
your left hand while you move the end of the pencil around in a
circle with your right hand, the pencil will tend to rotate in a
direction opposite of the direction you're rotating the end.
When you pedal a bike forward, the right pedal is moving in a
counter-clockwise (or anti-clockwise, on the other side of the
pond) direction, relative to the pedal shaft. This produces the
counter-intuitive (anti-intuitive on the other side of the pond)
effect of driving the pedal shaft in a clockwise direction --
rightie-tightie. For the same reason, English-threaded bottom
bracket cups