Rotation is often achieved by leaning forwards, not through true rotation. This tends to make the boat bounce in the water, and we all know that this is not good.

The aim of rotation is to all full use of your lat's. These are the biggest of the paddling group of muscles, and are the connection between the tip of your shoulder and your spine.

There are two good measures of shoulder rotation. Firstly, if you measure the distance to your catch (from the front of the cockpit) while your shoulders are square, then do the same again, however this time with pure rotation. Mark this spot on the boat. If you are entering the water past this point while paddling then you're probably leaning, and if you don't quite get that far then you're probably not rotating. In order to do this accurately you will need an observer (both on & off the water).

The second measure is more subjective, though just as important. You'll need an observer that watches you paddle from side on. Get them to make a note of the position of your shoulders at the exit of the stroke. The aim is that both shoulders can be seen clearly from the site. To help with this, place a coloured sticker on the top of each shoulder, near the tip of the shoulder. Both of these should be clearly visible to your observer at the exit of the stroke. This is easier said than done when you're fatigued! Note that very soon after the exit your shoulders will be blocked, so your observer will need to be on their game to help you here. Video can also help here.

As a drill, try sitting in your boat (IOP's can sit ON their boats), and place your paddle behind your head, and bring it down onto your shoulders. Test the degree of rotation you can get; and hold! You may need a helper to balance your boat.

Good luck!

The other articles can be found at Pillar 1: Posture and Pillar 2: Hand Position.

Other reading;
A Clean Exit.
A Good Hard Pull.