Watch for stair-step cracks in brick or block, doors and windows that stick or gap at one corner, sloping or bouncy floors, a bowing basement wall, or a horizontal crack in a block wall. Hairline cosmetic cracks usually do not need structural repair. A free inspection sorts cosmetic from structural.
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Not every crack means a foundation problem, and a good contractor will tell you when something is cosmetic. The signs that point to real structural movement fall into two buckets. Settlement signs (the foundation is dropping): stair-step cracks in brick or block veneer that are wider at the top, diagonal cracks running from the corners of doors and windows, doors and windows that suddenly stick or show a triangular gap at one corner, a chimney leaning or separating from the house, gaps where the wall meets the ceiling or floor, and floors that slope or feel bouncy over an interior footing. Lateral-movement signs (a wall is being pushed in): a horizontal crack across a block wall partway up, a wall that visibly bellies inward, stair-step cracks running toward the corners of a basement wall, and a wall that has rotated in at the top or slid in at the bottom. Cosmetic cracks, by contrast, are typically hairline, vertical or random, do not grow, and show up in drywall or stucco rather than the structural foundation. The reliable way to tell the difference is to measure: we use a manometer or laser level to map floor elevations and a string line to measure wall deflection, and for anything structural we bring in a licensed engineer to classify the failure before we propose a repair. Monitoring a questionable crack with a marked date and a gauge for a few weeks is sometimes the right first step rather than jumping to a repair.