Mesmer
It would be really cool to have! Unfortunately, Anet has made comments in the past (I can try to find them, but the search function on these forums is quite bad), where they say its not feasible with how the engine works right now.
I don’t think they ever went into details about why it won’t be possible, but they have told us they can’t easily do this.
I think a compromise is that when you cast portal exeunt, a “test portal” is internally deployed without using up your skill, if you are within range and you can use the portal, then the real one is dropped, otherwise if the “test portal” is out of range and is unusable, then your skill does nothing, or flashes red and doesn’t go on CD or something.
A request that has been raised time and again, because it makes sense. I hope they figure out a solution one day. In the meantime, this really belongs in the mesmer forum, and needs a better title. Seriously. “Mesmer”? That’s it?!?
An easier and completely possible solution would be to use the little red bar that other skills have under them when a target is out of range. Went past the working distance for the portal exit? Red line under the skill to let you know.