I found a very helpful post about exactly what I need here. It uses dot products to which is what I'm specifically looking for.
It talks about players facing monsters basically. They give a helpful picture here,
My question though, is what if you have a function with 3 arguments: the position of object 1, the position of object 2, and the direction object 1 is facing given as a unit vector. So the picture would look as so:
How would you find whether A is facing B?