Put on a shirt that is a very different color than your walking shorts.
Tuck the shirt in. This creates a roughly horizontal line between your
shorts and your shirts that is easy to see. If you wear the waist band
at or above the top edge of the hip bones (pelvic girdle), pull the
shorts down a little so the waist band goes across the top edge of the
hip bones. The goal is to have the line between you shorts and your
shirt to be on the top portion of your hips. Have a friend/spouse/coach
observe you from behind while you are race walking away from them. If
you have any hip drop, the observer should see the up/down motion of the
line between your shorts and your shirt. As a side benefit, you can
detect the dreaded hip sway (side to side movement) from these
observations too - just concentrate on the outside edge of either hip.

Hip rotation is easy to see too especially if you have access to a
treadmill. If you have a pair of shorts or warmup pants with a vertical
stripe down each side, wear those. Otherwise paperclip a small white
square to the waist band on each side of your shorts. Tuck your shirt
in, and position the waist band as described above for detecting hip
drop. Have a friend/spouse/coach observe you from the side while race
walking. Because your body stays in the same place while walking on the
treadmill, any forward and back movement of the vertical stripe or white
square is easy to see. You can still see it from a race walker going by,
but it will take a few times for a novice observer to see it, because
the race walkers body is moving too, and the hip rotation motion is
relative to the race walkers body. If the amount of forward motion looks
different than the amount of backwards motion, have the observer move to
the other side. This difference could be real, or it could be an
illusion. As a side benefit, you can see the relative sizes of the "in
front of the body" portion of the stride and the "behind the body"
portion of the stride.

If you do either of these on a treadmill, it makes it easier for the
observer, because the body of the race walker is not moving away from
the observer and the background objects are not changing. This gives the
observer a longer time to detect what is going on -- very useful for a
novice observer.

If the observer records what they see with a video camera, you'll be
able to see it too.