Cracks in masonry brick walls are difficult to evaluate by looking at them at a glance. This is because you need to determine if part of the wall system is still moving and why it has moved, which takes observation over a period of time. There are many reasons why a masonry wall will crack vertically--narrowing the reasons down is the difficult part.
Using a piece of paper and a pencil, sketch a rough drawing of your house or wall and mark all of the visible cracks you can find. Make note of their length and if they continue from the brick down through the foundation. Inspect the interior of the home and make note of any cracks in the finishes or doors that don't close properly. If you don't see any interior cracks, look for evidence of patching or repairs. Doors that don't close well can be evidence that a portion of the home has settled of heaved, which moves door frames out of alignment. Also make note of the width of each crack. Look at each wall from the side to see if bows out in the center, an indication that the wall's lateral restraint systems have failed.
Is the Crack Still Moving?
This is the most difficult part of an evaluation because you are trying to estimate future behavior by looking at a single moment in time. If a crack is still moving, it means your problem is going to increase. The age of the crack may provide some clues to this question. If the crack's edges are worn and/or there is debris accumulated inside of it, this may be evidence that the crack is very old and stable. A crack that has been recently patched and has since reopened is strong evidence that the crack is still moving. Special crack-monitoring devices can be installed on either side of a crack; these provide a more definitive answer to whether or not the crack is still moving and can tell you in what direction.
Why Has it Moved?
Determining why a crack has formed takes some experience and knowledge about construction techniques. Common reasons for cracks include: thermal expansion and contraction of materials, foundation settlement, tree root or frost heave, connection failure between two sections of the home and structural component failures. A vertical crack at the end of a masonry wall often indicates thermal or moisture expansion of the masonry. This can be a problem for long brick walls that were built without proper expansion joints.
What Are the Implications?
It is almost impossible to be definitive with the implications of brick cracks because it requires you to predict future movement. If there is evidence that the crack is still moving, if it is wider than a hairline and/or the brick is bowing or leaning, you should definitely have a specialist come in for further evaluation.