"...I dont know how to determine if there's a rotation on the clavicles..."

In a perfect human being the medial aspect of the clavicle or the sc joints will have perfect symmetry around the msp. Of course, I have never seen a perfect human being, but most people are the average human being. If the medial heads of clavicle or the sternoclavicular joints are equidistant from the median sagital plane, then there must by definition be no rotation. The question then is what constitutes the medial sagital plane?

Is it the thoracic spinous process? What about someone with severe scoliosis? Few people have perfect spines and rib cages - especially those that need spinal imaging. There will always be exceptions to any "rule" you find but i guess that's the interesting part of the job.

Comparing an observation of any upper thoracic structure with symmetry around the msp should suffice. The sc joints or the medial aspect of the clavicle are quick and obvious, allowing a reasonably accurate determination of relative distance as they are in the field of view simultaneously.

SCJ's equidistant from MSP -> Full marks unless you loose marks for contractions and spelling mistakes or failing to nominate the correct side (which you should).

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