Master Wu Chien Chuan passed this on that every move has to have Yin and Yang (not moving and moving) parts of the body and those are either in the torso (internal) or they are not (external). The moving and not moving parts form a "junction" called a Yin/Yang pair. One part of the "pair" moves and the other part does not move.
Much Tai Chi says it is "internal" but ask yourself the following question when you see it: Is the movement originating in the torso or is it originating in an extremity outside of the torso such as arms, legs, etc. ? You also have to reckon with the fact that any "extremity" is the furthest point or limit of something and that internal is always situated on the inside.