I think you can do this simply by checking the dot product between the normal vectors. If the dot product between the unit vectors along the 2 normals is 1 then the 2 normals are parallel else they are opposite.
Additionally, for the arrow, you can use Triad manipulator and display only 1 arrow.
Hope it helps!!!
I've never used Math Vectors, which looks to be what I'll need from looking up Dot function and Triad, but looks like it may be what I need. I'll just need to learn how to use it. I'll look into it later today hopefully and see if it solves my issue. Thank you.