Can healers tell if a team mate is out of range in Heroes of the Storm?

Pressing the corresponding ability will show the user a targeting circle -- anything within the circle is within range, and anything outside of it is not -- and the same is true of healing abililties.

Something I often do when playing as a healer is to "Pre-cast" my heals before an engagement, so that the ability is ready to cast and I have the targeting circle while dancing around the edge of combat.

If you're looking for something similar to how MMO RPGs handle out of range abilities (such as a small red dot, or greying out of the ability, Heroes of the Storm has no such indicators, at least in part because of how it handles unit selection). Instead, trying to cast on an out-of-range unit will move your hero towards the unit until they are within range, and then cast.

Of course, individual heroes / heal spells might change this - LiLi's healing spell, for-instance, auto-targets, and if LiLi herself is at full HP, the ability will gray out and prevent casting, rather than allow you to cast with zero targets.

With Lili you can see a target circle on the team mate with the lowest HP. With Lucio you can see the healing circle.

Aside from those two, I can't think of anyone else that would have a range indicator that you can see on quick cast.