We report a new type of optical biosensor capable of differentiating between bulk and surface perturbations of the ambient refractive index as well as between specific and nonspecific binding of molecules on the sensor surface. The proposed detection scheme is based on tracking the shifts of hybridized bonding and antibonding optical modes in coupled optical microcavities (photonic molecules). We demonstrate that by using two measurements of spectral shifts it is possible to discriminate between surface and volume index perturbation, to detect specific target molecules in a complex environment and to estimate the thickness of thin layers of adsorbed molecules.