Found this AVISynth Script in the AVISynth website documentation. Loads the source and completed target file, then enhances the difference
between the two in the windows below. This picture should explain it:



Just Save the Lines below as COMPARE.AVS , Modify it to reflect your file names, Load it in VirtualDubMod, and there you have it.

#------------Before and After Comparison Script----------------#

# If Videos start at different frames
frameadjust=0

# Videos to compare: (v1 is original, v2 is encoded or whatever)

v1 = AviSource("g:\apptest.avi").trim(frameadjust,0)
v2 = AviSource("g:\apptest2.avi")
sub = v1.subtract(v2)
substrong = sub.levels(122,1,132,0,255)

return StackVertical(StackHorizontal(v1.subtitle("origina l"),v2.subtitle("encoded")),StackHorizontal(sub.su btitle("Difference"),substrong.subtitle("Differenc e amplified")))

#------------END Before and After Comparison Script----------------#