How to indentify if the crack on plastered brickwalls is cosmetic or structural?
A good way to determine the seriousness is to call an engineer, as you have done.

Because soils can settle, water can degrade materials, concrete and other materials can expand and contract over time, storms exert wind forces, and seismic events occur, design and/or construction  issues may express themselves in the structural system at any time. So may cosmetic issues.