Dear Jonathan, the big divide is between hand-made paper and machine-made paper. Hand-made never has a water mark. Machine-made quite often does. Also, MM has chain- and laid-lines, regular parallel lines that you can see quite easily if you hold the paper up to the light. Also, MM paper rolls, or folds, more easily in one direction than at right angles to that direction (because of the difference between chain- and laid- grain); HM paper bends the same in all directions.
There do exists indexes of watermarks, but I'm afraid I don't recall the details now. I kept track of watermarks when I was cataloguing the Wellcome Library Sanskrit collection in ythe 80s. If you get hold of my "Handlists" and do a keyword search, you'll find them. I didn't make an index at the back of the book, though. For example, in Handlist I (1985),
serial number 672 (p. 165) is watermarked "Brittania".
#683 is "Dorling & Co. London",
#698 is "Conqueror, London",
#774 is "Fine Foolscap Balmukand Ramji Das",
etc.
Best,
Dominik