The longitudinal spin Seebeck effect (LSSE) and the anomalous Nernst effect (ANE) are investigated in various metal/insulator junction systems and a clear separation of the LSSE from the ANE induced by static magnetic proximity is demonstrated. This separation is realized by comparing transverse thermoelectric voltage in in-plane magnetized (IM) and perpendicularly magnetized (PM) configurations, where the LSSE appears only in the IM configuration while the ANE appears both in the IM and PM configurations. We show that, in Pt/Y 3Fe5O12 samples, the LSSE voltage in the IM configuration is three orders of magnitude greater than the proximity-ANE contamination estimated from the data in the PM configuration. This quantitative voltage comparison between the IM and PM configurations is corroborated by systematic voltage measurements in Ni81Fe19/Gd 3Ga5O12, Pt/Gd3Ga5O 12, Au/Y3Fe5O12, and Au/Gd 3Ga5O12 samples and by our phenomenological model calculation. The LSSE measurements in high magnetic field regions further confirm that the observed voltage in the Pt/Y3Fe5O 12 and Au/Y3Fe5O12 samples is of magnon origin. We apply this voltage comparison method also to a Ni 81Fe19/Y3Fe5O12 sample and show that both the LSSE and ANE exist in this sample. © 2013 American Physical Society.