How to know if a dihedral angle is + or -?

Question

In several posts post1 and post2 have been shared how to obtain a dihedral angle with four points or their two planes. But ... How can one know if that dihedral angle is positive or negative?

• What I have in mind to clasify them is the following diagram:

Answer 1

Calculate triple product of vectors P1P2, P2P3 and P3P4.

Sign of the value of this triple product shows handedness of vector triplet (right-handed or left-handed) and corresponds to the sign of dihedral angle