How do I make enemy HP visible when they are in middle of my screen?

I have a circular crosshair in middle of my screen. I’ve attached a HP bar on actors in the game world that always faces the player. I want those HP bars to only show up when the player is aiming the crosshairs at the actors. I’m not entirely sure how to set that up. I figured I could use a line trace but that wouldn’t work in far distances as the crosshair (camera frustrum?) would slowly get bigger than a normal line trace.