basic trig. Given right triangles and finding the 3rd angle, use the law of sines to find the shortest side (the drop). The 6 degree stem will have a drop of 9mm while the 10 degree stem will have a drop of 15mm. Subtract 1mm from the 10 degree to compensate for the stack height, and the 6 degree stem is still 5mm higher.

Now this doesn't account for the head tube angle, in which case you would have to rotate the entire equasion about 15-20 degrees to know the actual vertical drop. But that doesn't change which is higher, just by how much it is higher, and even that would be a small variance.