And you can find the answers in the sixth edition of A Pocket Guide to the Identification of First Editions. It lists over 3700 publishers from 1850 to the present and the manner in which they have identified the first printings of their books. This is the largest edition ever, with over 600 publishers added to the fifth edition and many previously-listed publishers clarified and explicated based on added research. No other guide to first editions is as comprehensive, or as easy to use.
Publishers from the United States, the British Empire/Commonwealth and other English-language publishers from around the world are included.
You'll know exactly what to look for on the copyright pages of books you are examining. And whenever possible, you'll learn when a publisher changed methods of identification: many did, some several times in their publishing history. Other guides to first editions merely list the publishers' statements about first edition identification; the editor's scholarship is based on statements compared with thousands of personal examinations of books by publishers over the full span of their existence to determine consistency and changes of methods.