How can I differentiate between a minor crack and a major crack that requires immediate attention? 

Minor cracks are usually less than 1/8 inch wide and don’t show any signs of structural problems. They are often vertical and do not extend the full length or height of the wall. Major cracks, on the other hand, are generally wider, horizontal or diagonal, and could span significant portions of the wall. They may be accompanied by other signs of structural damage, such as wall bowing or uneven floors.