I got hold of a Flatcap XS and wired it up right to my Nait XS Amp. I turned on the Flatcap XS first, then the Amp (don’t really get why the new external power supply isn’t now the ‘power source’ for the amp and you have to still turn both on).
It isn’t exactly obvious it is doing anything. E.g. a little LED saying ‘external power supply working’ on the Amp would be a nice touch. How can you tell? Obviously each device is on from the Naim glow, but that’s not proof they are working together.
I know ‘listening’ is one obvious answer. But I’m suspicious of subjective analysis and it hardly seems the scientific way to tell something is on and working as it should.