This document explains and demonstrates an experimental method to determine principal plastic strains in industrially stamped sheet panels. The principal strains distribution after a given stamping process can be obtained using computer aided grid experimental method. In contrast with FLD (Forming Limit Diagram) obtained by the material testing, the measured results of strain distribution can be used to determine the sheet metal's formability allowing to determine at which point the sheet metal cracks or uneven stretch occurs and other forming defects. The main principle and related theory of this approach are discussed. One automobile panel stamped part as a practical case was studied, the strain distribution of the part after a given stamping process was measured and calculated, a demonstration of how to deal with the results in comparison with FLD to determine and solve forming problems is analyzed.