Measurement of misalignment in Stitching

I stitched two pictures with an alpha blending. Now my question is: is there any way to measure the quality of the stitching result. My idea is to compute the difference between the two images in the overlapping area and then sum all pixels in this area. A high value for the sum would be mean that the result is very bad.
Is there any better way to measure the quality?
Thanks in advance