I must start be stating my position that perplexion doesn't necessarily imply abnormality. There.
Off the top of my head, I know of two ways (I'm sure there're more) to do what you are asking about:
1. Premium Trading Platforms. Some (costly?) trading platforms have tools designed to identify or help to identify sector rotations. At least this used to be the case when I used them.
2. Identify indices that correspond to the sectors you are interested in. Using Google Finance (or equivalent), plot these indices, along with the S-Fund underlying, using the comparison graph (uses percentages rather than absolute values)--as many at a time that your brain can visually discriminate. You will look for a proportionally inverse relationship between the underlying and one or more of the sector indices. (In other words, as one goes up, the other goes down.) If you find that, then you may have spotted 'rotation.'
Whether the underlying is leading or lagging may also be evident. (Goes up before the others and vice versa.)