If your visibility on a square changes, either because your visible range decreases (Mists of Chaos, removed Necklace of the Eye), or because you backed up, I know of no way to tell whether a square appears empty because it IS empty, or because it is out of my visible range.

This is most often helpful when trying to teleport a short distance, but could be helpful in other ways.

A way to identify these squares would be situationally very useful. It doesn't need to be fancy, even just a modified 'l'ook command that told me if a square was in range would be nice.

[EDIT: Jellyslayer pointed out that the 'Change Highlight Mode' command, bound to 'H' does exactly what I would want this to do, and more than I asked for. So this is an unnecessary RFE.]