How to confirm external reference is valid on AD7175-8

I'm planning on using an external reference with the ad7175-8 and I have a system requirement to monitor the reference to ensure the conversions are correct.  I noticed that the crosspoint mux allows for the inputs to be connected to the ±REF pins directly.  When this occurs, how is the conversion made?  Is the external reference used for the conversion or is the internal reference used to sample the signal.  

The reason the distinction is important is so that the actual failure is easier to identify (i.e. if the converter is faulty or if the external reference is faulty).

The other option I'm considering is to sample another fixed reference voltage to one of the available analog inputs and sample that signal to determine if there is a reference failure or a converter failure.  This is what we have done in the past but I'd like to save myself an available analog input if possible.