I use this technique to get web layouts pixel-perfect with the mockup, just put both windows next to one another and superimpose them with your eyes. Works great. There are tools that do this by overlaying an image with 50% alpha but it doesn't work as well.

Last year when there was a bunch of fuss about Kate Middleton not having made any public appearances there was a minor flap where people claimed that a photo she'd released was just an edit of an earlier photo.

There was a tweet presenting two photos, one old and one purporting to be new, where she was holding strikingly similar poses. The claim was that the new one was just an edit of the older one. I used this technique and immediately the minor differences stuck out like a sore thumb- her hand was rotated more in one, her hair was laid differently, etc.