Find the slopes and their product should be - 1 for the diagonals to be perpendicular.
The points p and q have zero y-coordinate, so they are on the y-axis.
The points r and s have same y-coordinate b, so they are on the parallel line to the y-axis.
The points p and r are opposite to each-other, similarly points s and q are opposite points. So the diagonals are pr and qs. This is also known from the order of vertices in the name of the quadrilateral.
Slopes are:
The product of slopes:
The diagonals are perpendicular if: