Single Point Incremental Forming (SPIF) is an emerging sheet-metal-forming technology, capable of manufacturing complex parts at low cost for small to medium-batch production. The present paper is focused on presenting an innovative and viable method to test the thinning limits of sheet metals in Negative Incremental Forming along with verification of the Cosine's law of thickness distribution. The Cosine's law was verified by comparing the experimentally measured thicknesses of incrementally formed parts with those predicted by the law. To test the thinning limit of a sheet metal, the idea is based on the forming of an axi-symmetric part varying its slope with depth corresponding to varying thinning. An arc of a circle was selected as generatrix to model such an axi-symmetric part. Based on the Cosine's law, mathematical expressions were derived to predict the thickness distribution along the depth of the part and the thinning limit of the sheet-metal. The Aluminum sheet metal was used as an experimental material. In order to test its thinning limit, the axi-symmetric part, modeled with a generatrix arc, was formed incrementally until it cracked. Thickness of the fractured part was measured at various points along its depth and compared with that predicted by the Cosine's law. The maximum thinning at a point, at which thickness followed the Cosine's law, was called the thinning limit of the sheet metal. In order to obtain accurate results, four such parts having the same generatrix design were formed. Based on these results, several axi-symmetric and asymmetric parts were formed at fixed slopes. It was found that the thinning limits obtained from the parts formed at fixed slopes were a little lower than those obtained from the parts modeled with the same generatrix design. As conclusion, a strategy to test the lowest possible thinning limits of sheet metals has been proposed. The proposed method is capable to test the thinning limits of sheet metals at reduced processing time and cost.