Thermoelectric materials have the ability to directly and reversibly convert waste heat into electrical energy. Thermoelectric energy conversion could play an important role in determining the future of our energy utilization scenario provided we achieve a competitive energy conversion efficiency with low-cost and environment-friendly materials. This can either be achieved by improving the conversion efficiency of existing materials with new electronic structure modulation and phonon transport optimization strategies or by finding new thermoelectric materials. However, both the process of finding a good candidate thermoelectric material and evaluation of the applicability of a new strategy requires extensive measurement of electrical conductivity, Seebeck coefficient, and thermal conductivity. In our work, we show that optical dielectric function of a material, which can be obtained relatively easily and rapidly, is closely linked to its electronic and thermoelectric transport properties and can be taken as a reliable indicator of its thermoelectric performance. We show that both the position and intensity of the imaginary part of the dielectric function of SnSe and GeSe changes as the chemical bonding and crystal structure evolves with doping and alloying. We establish that the optical dielectric function along with the recently developed concept of quantum mechanical description of chemical bonding in a solid, based on electron sharing (ES) vs. electron transfer (ET) between the adjacent atoms, is a powerful tool to design high-performance thermoelectric materials.