hate to ask, but how do I know when an ally is hexed?

how do I properly use Shatter Hex? I've only got this skill to work maybe 2 out of 30 tries by pure luck/fluke by casting it on warrior allies when their health is going lower.. there must be an easier way to tell if they are hexed lol
do I look on the party list and see their health bar change color? any tips

TIA (fairly new to game)

There is a down purple arrow on the ally's health bar.

No more point to this thread.