To properly measure the VESA pattern on the backside of you monitor, you will need to find the distance in millimeters of the screw holes horizontally and vertically.
To find your first measurement, you will need to measure the horizontal distance between the centers of the left and right side holes. To find your second measurement, measure the vertical distance between the top and bottom holes.
An example of the VESA pattern may be 100x100mm or 600x400mm.