How can I determine if an enemy is within range of a skill?

Solution 1:

If you want to know the range of your skill, it's located on the tooltip. But knowing the numerical value is only useful for estimating whether you might be in range. To know if you are, you need to look at the icon for the skill itself.

By default, skills which are out of range will have a red bar along the bottom of the skill icon. If this bar is present, you are out of range, and your attacks will "Miss".

Do note that this is only helpful at determining raw distance; obstructions may be present that prevent you from actually hitting your target, even if it is in range.

In the above image, you can see that the Dagger skills for 1 and 2 are both out of range, while the 3rd skill is not (in this case, because it's a self-buff).