If I remember correctly, if the tertiary shaft seal (outboard side) is not holding well, you would see an abnormally high flow of inert separation gas on the compressor shaft seal housing integral with the compressor casing.
If the inboard side of the tertiary seal is not holding well, you would see high flow of sep gas to the atm via the local vent, which you can pick up with a PSH and restriction orifice on this local vent.
