I currently know of only one way of determining if you are in range to use a skill, and that is the red bar underneath the hotkey or a red area marker.
For skills that have damage that varies by distance, i.e. Engineer’s Bluderbuss which has a max range of 400 yet max damage at 100, it would be nice if the range indicator could change color based on how far you were.
Just a thought would be to have different color indicators other than red and nothing, say a yellow indicator so you know that you are not within 100, but still within 400.
Even a number that says > or < 100 would be nice.