I don't know how one can do that but to compare screenshots.

One thing you can do is do grabs of the application window rather than the screen. In windows terms, Alt-PrintScreen rather than Ctl-PrintScreen. The advantage is that you remove the absolute position of the window from the equation.

An alternative is to capture the api/parameter sequences as they are passed into the device driver by using a device driver that logs this information or by interjecting dummy that logs and does pass-thru to the real thing. As I said, this is only really useful for testing window managers/gui libraries themselves and not applications.

Here I can spot another trap:* The test machine has some "skins" enabled (depending on the Widget Toolkit)

You run your testing on a "standard machine".