How to detect if entity is looking up/down?

So I need to be able to detect if the player is looking up or down. I figured I could use the block for "if entity is facing (north/south/east/west/up/down)", but while it works fpr the four cardinal directions, it doesn't seem to be able to detect if the player is looking straight up or down. Maybe this is because they are still "facing" east even though they are looking down? If that is the case, then what does it even mean to be "facing" up or down, and how can I detect if the player is looking up or down?