Width, direction, and movement. Vertical hairline cracks under a sixteenth of an inch are usually shrinkage. Horizontal cracks, stair-step cracks in block, cracks wider than a quarter inch, and any crack with offset between faces warrant evaluation.
More detail
Foundations crack. Concrete shrinks as it cures and almost every poured wall in Greater Cincinnati and Northern Kentucky has vertical cracks somewhere. The useful question is not whether there are cracks but which ones indicate movement. Direction first. Vertical and near-vertical cracks in poured walls are usually shrinkage and are the least concerning category, even when they weep water, which is a sealing job rather than a structural one. Diagonal cracks running from the corner of a window or door are stress concentrating at an opening and are often benign, though they warrant watching. Horizontal cracks are the serious category: a horizontal crack along a mortar joint in block, especially in the middle third of the wall height, means lateral soil pressure is pushing the wall inward. Stair-step cracking through block mortar joints usually means differential settlement, one part of the foundation moving relative to another. Width second. Under a sixteenth of an inch and stable is generally cosmetic. A quarter inch and above deserves professional evaluation regardless of direction. Any crack where the two faces are offset, meaning one side has moved out of plane with the other, indicates displacement rather than shrinkage. Movement third, and it is the one that settles the question. Mark both ends of the crack with a dated pencil line, measure the width at a marked point, and re-measure in three months. A crack that has not moved through a full wet and dry cycle is a very different problem than one that has widened. Greater Cincinnati sits on glacial till over Ordovician limestone and shale, with clay-rich soils that hold water and hillside slopes that concentrate it. That is why seasonal moisture change drives so much of the movement seen here, and why the same crack can look worse in a dry late summer than in spring.