Resistance spot welding is one of the mostly used methods in joining different pieces of complex sheet metal structures such as automotive BIW. Some manufacturing inaccuracies, accessibility difficulties to some locations and large quantity of spot welds, may lead to missed out substantial number of spot weld connections. In present work, it is tried to find suitable Indexes to detect the locations of omitted spot welds in a structural joint using finite element method and modal analysis. Two different special modal indexes derived that can be used to detect the missed out location of a spot weld in a lap joint of two flat plates. One modal index derived from computations on high range natural frequencies of the structure and the other one was achieved by searching on a special case of MAC values of modal vectors at joint region. Finally, using artificial neural networks and the developed indexes in present work, the location of an absent spot weld in the investigated structure detected successfully.