Resonance energy transfer (RET)- or protein-fragment complement assay (PCA)-based biosensors useful for assessing the activity of G-proteins are described. These biosensors are based on the competition between the Got subunit and a Gβγ interacting protein (βγ IP) for the binding to the Gβγ dimer. These biosensors comprises (1) a βγ IP and (2) a Gβ or Gγ protein; a GPCR; or a plasma membrane targeting domain, fused to suitable RET or PCA tags. Methods using such biosensors for different applications, including the identification of agents that modulates G-protein activity or for the characterization of GPCR signaling/regulation, such as G-protein preferences and activation profiles of GPCRs, are also described.