When you are preparing to change lanes on a roadway, or turn at an intersection, you must check your blind spot by?

Blind spots signify the regions to the sides of the car, which cannot be seen in the side mirrors or the rear mirrors. Thus, to ensure that these spots are clear prior to changing lanes, one has to turn around physically and look to witness what thing is happening outside.  

Thus, all it takes is a mirror check and a shoulder check to ensure that one is safe to move over.  

obviously, the latter.