Examine the crack location and the course of the crack through the wall to determine the likely cause of the damage. Cracks located at the base of the building or following horizontal lines across the face of the brick walls are likely signs of bad cracks that can lead to eventual failure of the brick.
Check cracks at the base of the walls. If wider at the crack top than the bottom, they may be subsidence cracks, resulting from underground activity. These cracks often occur at the corners of the walls or run the full height of the wall. If the crack is wider at the base, the cause is likely ground heave, resulting in movement of the foundation of the wall. They're less dangerous than subsidence cracks, but can still lead to structural damage.
Examine horizontal cracks throughout the mortar for metal ties evident within the crack interior. If present, the crack is a wall tie failure. The wall ties hold the brickwork in place, and with failure can release the skin of brickwork on the outside of the wall.
Determine if the crack follows the path of mortar along the bricks, usually in a zig-zag pattern. Examine the space between the bricks and compare it to the spacing of intact mortar. If the spacing is wider, then the zig-zag crack is likely the result of expansion.