Since 2012, it has been a requirement of the ANSI/ASSE Z359.14 standard that all self-retracting devices must include a visual indicator that the device has been subjected to a fall. In many cases, this is a simple roll-pin or shear-pin that is part of the snap hook assembly. In the event of a fall, the pin shears off, revealing an identifying collar or mark.
For devices featuring a webbing constituent line, the visual indicator may be a simple warning label or flag. It is stitched to the webbing near the nozzle hook at the bottom of the housing and concealed in a section that has been folded and stitched down. The force of the arrest breaks the stitch and reveals the concealed warning label.
Check manufacturer instructions for your preferred device for details on the visual indicator and how to determine the go/no-go criteria for your device. Any device that has been involved in a fall cannot be reused.