Make a high resolution scan, 1200 dpi if you can. Then look at an edge, like of a numeral or a frame. The litho will be more or less smooth and even. The photo will show dots along the edges.
Check out the excellent USB microscope pictures in this Stamporama thread posted by sheepshanks: https://stamporama.com/discboard/disc_main.php?action=20&id=15799#118479
These are on Hong Kong machins, not Great Britain machins, but it is the same thing that Ted is trying to say, only a picture is worth a 1000 words. This was an outstanding, classic post by Ningpo and sheepshanks. I sure miss Ningpo.
