The Dungeon Master's Guide has enjoyed numerous printings. The earlier printings are difficult to identify and interested parties should definitely consult expert resources when purchasing or investigating specific, early printings (even the experts do not always agree on the details). Again, there is no simple or easy way to unambiguously determine an early printing except through careful scrutiny and reference to an expert source. Even the term "printing" is ambiguous, as in some cases it is known not to coincide with the physical act of printing batches of books (the term Printing is retained by collectors because the product itself uses it).
The 1st - 7th Printings feature the "old" cover of three adventurers fighting an efreet.
The 1st, 2nd (Alpha and Beta), and 3rd (Alpha, Beta, Gamma) Printings have 232 numbered pages, a wizard TSR logo, and yellow-orange endpapers. Some 2nd Printing copies erroneously were bound with portions of the Monster Manual; some 3rd Printing copies have broken covers due to a boxing error).
1st Printing - August 1979
2nd Printing - August 1979
3rd Printing - About September - November 1979
The 4th Printing has 232 numbered pages, a wizard TSR logo, and white endpapers; it also includes an ISBN.
4th Printing - About September - December 1979
The 5th Printing has 232 numbered pages, a wizard TSR logo, and yellow-orange endpapers
5th Printing - About September - December 1979
The 6th Printing incorporates revisions (described in Dragon (Issue 35 - Mar 1980)); the title page notes "Revised Edition - December 1979" (this inscription remains throughout at least the 8th printing); it has 238 numbered pages, a wizard TSR logo, and yellow-orange endpapers - however, later copies from the sixth printing have white endpapers and 236 numbered pages.
6th Printing (Alpha, Beta, Gamma) - December 1979 - 1980
The 7th Printing has 238 numbered pages, a face TSR logo, and white endpapers.
7th Printing - 1981
The 8th and later Printings have an orange spine and revised cover art of a wizard opening double doors (though the back cover blurb continues to refer to the old artwork throughout the lifetime of the product).
8th Printing - 1983
Beginning in 1985 (apparently with the 9th or 10th Printing, though this needs further scrutiny) the copyright pages show the Printing information. By this point, Printings had stabilized into large volumes and fairly predictable patterns.
9th Printing - August 1987
10th Printing - 1987
11th Printing - April 1988
12th Printing - November 1988
13th Printing - July 1989
14th Printing - July 1990
Subsequently, "premium" reprints of the book have been made available though they are all clearly marked at such.
Exhaustive information, including a diagnostic flowchart, can be found at https://www.acaeum.com/ddindexes/setpages/dmg.html