Supra-thermal (>100 keV) electrons generated by laser plasma interactions can be detrimental to the performance of ignition experiments conducted on the National Ignition Facility (NIF). On a NIF shot, the amount of electrons is estimated by measuring the hard X-rays passing through the hohlraum wall. The primary sources of hot electrons in a hohlraum are Stimulated Raman Scattering (SRS) and two plasmon decay (TPD). While SRS is well diagnosed on the NIF, there has been no diagnosis of TPD. We have designed and implemented a new diagnostic to characterize the time history of TPD on the NIF. The instrument provides a time resolved measurement of the 3/2 ω harmonic emission which is indicative of the presence of TPD. We describe the diagnostic setup, calibration, and the preliminary results obtained on NIF hohlraum experiments. We find evidence of a correlation between measured hard X-rays generated from the hot electron bremsstrahlung and the TPD emission.