There are a number of visual symptoms that can be seen when brick and stone masonry structures need restoration.
Restoration of an existing stone or brick structure may only call for replacing a few masonry units that have been damaged by accidental blunt force impacts that require grinding out the mortar surrounding the damaged units, removing the damaged units and re-pointing them back into place with a matching mortar.
Then there are times when more serious restoration processes need to be undertaken, when portions of a masonry structure begin to show discoloration, brick face spalling (cracking and chipping), mortar cracks and crumbling. These are signs that the masonry structure is under siege from the elements from the inside and the material of the wall begins to break down. This type of damage is usually caused by moisture getting inside the brick or stone face structure and gets trapped there. Then when freezing and thawing takes place, the material expands and contracts at different rates, causing a breakdown in all of the material, brick and mortar alike. This cause of this kind of deterioration needs to be stopped by finding the source of the leaked moisture, whether it be a leaking wall flashing or wall cap etc… Once the moisture leak is found and stopped, then there are a number of ways to approach the repair or restoration depending on the extent of the damage. If the extent of the damage is large enough, some times you would be better served (cost and time wise) to just replace the entire wall or structure.
When an unstable foundation or a foundation failure is at fault, you will see the mortar on a brick or stone structure begin to pull away or separate from the brick or stone units, leaving gaps between the mortar and the masonry units in a horizontal stair step pattern. There can also be present, occasional vertical cracks in the mortar and sometimes even in the brick units themselves. The foundation will need to be repaired before any effective restoration can take place.
Another note: if you are planning to have new exterior hard-scape work done such as, free-standing masonry walls, fireplace, outside entertainment areas and retaining masonry walls, you must (must) have a properly poured, re-bar reinforced, concrete footer underneath the masonry structure, to have a sound, long-lasting masonry fixture.