Detect if player is looking behind character?

I'm making a head-turning system where the character's head faces the camera look vector and yada -yada. The script works perfectly, the head-turning part at least, but I need a way to detect if the player is looking behind their character and not forward. I've tried to use Dot and other CFrame and math functions but none of them worked. So how could I theoretically detect if a player is looking behind or forward to their character?