Only a structural engineer or mason can say for sure, but if yours shows any of these signs, it may be time for a new one.
Walls like the one shown, that slant noticeably are not long for this world. Whether it's caused by tree roots, poor drainage, or a failed footing, a leaning wall will probably have to be demolished and rebuilt from scratch.
You can fill minor blemishes in poured concrete with hydraulic cement. In a mortared wall, simply chisel out damaged joint filler and repoint. But if the cracks are more than ¼-inch wide and deep, and more than 2 feet long, the wall may have structural damage. Call in a pro to assess whether the affected area can be repaired or has to be replaced.
A pronounced dip in the courses of a timber or interlocking block wall indicates that the footing has failed in one spot. A pro may be able to replace the footing beneath the damaged area and rebuild just that section of the wall.
When mortared masonry develops a pot belly, the likely culprit is a buildup of water pressure behind it. Try drilling a ½-inch weep hole with a masonry bit to encourage drainage. A bulge in a timber or interlocking block wall is typically caused by a lack of anchoring. Careful excavation behind the wall may allow a protruding section to be rebuilt. Consult a pro before starting work.