Rene Ritchie for iMore, on how to discern which state the shift key is in:
If the background is dark gray and the arrow is white, you’re in lower case mode.
If the background is white and the arrow is black, you’re in Upper case mode.
If the background is white and the arrow is dark, and there’s a horizontal line beneath the arrow, you’re in ALL CAPS mode.
The dark keyboard’s better contrast makes distinguishing this much easier. That said, the current implementation isn’t the best design decision by Apple’s UX team.