Well... For this particular one you could do an extrude cut to the mid plane and then get the largest face...
But yes, you should be able to check all adjacent faces. I believe first you would get the normal vector of the selected face. Then get the outer loop, then all edges of the loop, then the co-edge of each edge, then the face that the coedge belongs to. Check to make sure the face is planar or cylindrical (the two types of faces that could be 90° from the other face), get a normal vector, and dot it with the original vector you had. If the dot product is zero, the vectors are orthogonal.
Yes for this one you are correct you can do a mid-plane cut but I cannot guarantee that will always work, i.e. tapered counterbores into will have a different thickness depending on where they are cut.
I am going to have to do some reading up on vectors as a refresher because I have not used them since leaving university. Thanks for the advice though.