Age of house? and Is the CMU inside the crack bright and clean looking? or dusty and dull? Dusty and dull means its been around a while and is likely initial differential settlement - not an issue. Clean and bright usually indicates more recent settlement - could be an issue - depending on age of house. Not foolproof assessment, but a type of rule of thumb.

Is the brick crack directly in line with CMU cracks? Minor cracks at different locations typically not as bad as inline cracks through multiple building components.

Check doors and windows in first floor directly above cracks. Do they operate freely? or do they stick?

Are there cracks in concrete floor that align with CMU cracks? could also indicate problems. Look close - could be hairline depending on age of house.

Minor cracks in masonry foundation walls are USUALLY not a big issue in residential construction.

If house is 10+ years old and the crack is hairline width - it is unlikely it just started and probably will not get worse. Minor hairline cracks often occur in residential masonry walls soon after construction and never get significantly larger. Level of workmanship and usual lack of control and/or expansion joints are often cause of these cracks in residential work.

Look for any previous repairs around crack. If there is sealant or different colored mortar - and it still has cracks - this would be an ongoing problem.

Again every situation is different, just some eye test things to look at. Good Luck.