1. A method comprising: directing an end effector of a robot to lay up tows of material based on a Numerical Control (NC) program; acquiring images of the tows as the tows are laid-up by the end effector; receiving input from a measuring device as tows are laid-up by the end effector; detecting a pause by identifying a set of consecutive pulse signals in the input that are separated in time by an average period that is greater than a threshold; and updating stored data to correlate the images with instructions in the NC program, based on the detected pause.