I need my back/lower back to be as strong and immune to injury as possible for my work. How do I tell if it's there or close?

Strengthening. If your back is in good condition and have not given you any problems, the best way to keep it healthy and strong if to strengthening of the core and para vertebral muscles, do not get heavy, starch it regularly, daily intake of ca and vit d.Learn proper mechanics of lifting and know your limitations.