Key indicators include: horizontal cracks in basement walls (lateral soil or hydrostatic pressure — structurally evaluate these); stair-step cracks in brick or CMU walls (differential settlement); diagonal cracks from corners of windows and doors (differential settlement or seismic movement); doors or windows that stick, bind, or no longer latch (foundation movement distorting the frame); separation between foundation wall and sill plate; floors that slope visibly or feel springy; water infiltration through the wall or slab; and efflorescence — white mineral deposits from water migrating through the concrete. Not all cracks indicate structural failure. Hairline shrinkage cracks in poured concrete walls are normal. Any crack wider than 1/4 inch, growing, or showing horizontal displacement requires structural engineering assessment before repair, because the repair must address the cause, not just the symptom.