As the curved plates of the Rotary press made the stamps slightly larger it is relatively easy to discern which stamp is flat plate and which is a rotary press stamp. First select any perf Washington Franklin stamp or the first issue Washington Franklin 1 cent or 2 cent. These are the stamps with the numbers one and two spelt out, instead of numbers being displayed. I chose the latter alternative as shown in the first image above.

Then cut out squares at each corner. As shown in the second image above. Placing the stamp you wish to test under your cut out stamp you can see if the frame lines match. If, as in the last image shown above the frame lines are outside the top stamp in either the top, bottom or sides then you have a rotary stamp. If the lines are in the same place, as shown in the third image, you have a flat plate stamp.

This test works with any value stamp.

The image above is a perfect example of the reverse of a flat plate stamp. The flecks of carmine ink on the reverse can be found on flat plate and are very rare on rotary press stamps. The cause of the flecks of carmine ink is that during the flat plate process the sheets were placed on top of each other before the ink had a chance to dry properly.