Animal cells are active, contractile objects. While bioassays address the molecular characterization of cell contractility, the mechanical characterization of the active forces in cells remains challenging. Here by confronting theoretical analysis, finite element modeling and experiments, we analyze the relevance of existing mechanical approaches and show that their complementarity allows us to characterize both the resistive and the active components of the intracellular stresses that build up following cell adhesion. By highlighting their linear correlation, we show that these mechanical approaches allow us to localize the stress generators in the cells, give an upper bound to their size, and demonstrate that they are mechanically linked to the substrate at a very local scale.