Checking that two adjoining pattern pieces are the same length, measuring at the seam (not the cut edge).
In this case, we are checking that the back side seam length (A to B), is the same as the front side seam length (A to B plus C to D).
I am starting from the waist level moving upwards. Where the front side seam dart point B is reached, a point/line is marked on the bodice back.
The Bodice Front is moved so that the bodice front point C aligns with the mark previously made, and the line length C-D on the bodice front is checked against the remainder of the line length on the bodice back.