Frequently stamps are offered on eBay that seem to have perforations so irregular the suspicion arises that they have been faked. There is a very simple test one can perform with the image of such a stamp called a "flip-comparison" test, that can be done with a program such as MS Paint. This tutorial will guide you through the steps of such a test, using MS Paint. Other programs may vary slightly in how they perform the tasks illustrated below, but interpretation and adaptation shouldn't be too difficult.
    First, copy the stamp's image to your hard disk. Right click on the image, and select "Save picture as", then choose a place, such as your desktop, where the image will be readily accessible.
    Next, open the image in MS Paint, or similar program. Use the SELECT tool (highlighted at top left in image below) to outline the perforations on one side of the stamp as shown by the area enclosed by a dashed line.

Next, copy this selected portion of the image. You can either use the EDIT menu or the keystrokes CTRL+C. Once this portion of the image is on the clipboard, paste it back into the image, either using the EDIT menu or the keystrokes CTRL+V. Your image will now look like this:

A copy of the perforations you selected now appears at the top left side of the image. Now, with the copy still highlighted,
flip it both horizontally and vertically. This is done via the IMAGE menu, shown below.

Once you select "Flip/Rotate" you will be given the options "Flip horizontal" and "Flip vertical". Remember that the image must be flipped both ways, so you will need to open the IMAGE menu up twice and perform each flip separately. What you have then will look like this:

The image within the dashed lines can now be moved up against the side from which it came, and the perforations compared.
In the illustration below, the perforations match very well, indicating that that side of the stamp was not reperforated.

And below is a stamp that did not pass the test:

Not only do the perforations not line up, they are not on a straight line.
This is very much a red flag!