To determine if a concrete crack is active or dormant, you can use the following methods: 1. Measure the crack at regular intervals using tools such as Demec gauges, Tell Tale crack monitors, handheld measuring microscopes, marker pens, or tape measures. 2. Active cracks are at risk of growing and causing more damage, while dormant cracks are stable but still require sealing and waterproofing. 3. If the crack shows signs of growth or appears to be widening, it is likely active and should be repaired promptly. Consult a structural engineer for further evaluation.