I haven't tried this, so someone else please correct me if it won't
work, but why not color the sides and end of your tenon with chalk or a
soft-lead pencil, try it in the mortise, and see where the chalk rubbed
off most inside the mortise? Seems to me like it should work. In the
past I've always been able to figure out which sides need more chopping
or paring by just wiggling the tenon piece and noting where it feels
like it is stuck. Does that make sense?
Good luck,
Andy