"How does the user know which sample rate is being streamed from Amazon?".
I don't understand. If you click on the Ultra HD square it tells you. It also tells you what your device is capable of. So if I play from my iPhone using the Apple dongle it recognizes it's only capable of 48/24. If I plug in my Dragonfly it recognizes it's capable of 96/24. And it always says it's playing at the resolution it was downloaded at--unless it's 192/24 and it has to downgrade it to my device's upper capability. So if I download a 96/24 album it says it's playing at 96/24 with the Dragonfly but only 48/24 when I rely on Apple's dongle or my Bluetooth headphones.
I'm not an audiophile expert, but I'm not understanding the issue. Unless Amazon is lying, aren't these tracks already playing at the resolution they are supposed to be playing at?