A method is proposed to identify the near crack-tip region in a deforming object as the plastic zone and to diagnose its status as to whether or not the crack is about to open. The in-plane displacement field is visualized as a two-dimensional, full-field optical interfero-metric fringe pattern, and the diagnosis is made based on the plastic deformation and critical fracture criteria derived from a recent theory of deformation and fracture called physical mesomechanics. The proposed method is demonstrated for tensile experiments conducted for tin and steel specimens.