Collectors should be aware that there was an EXACT REPRINT of SUPERMAN COMICS # 1,  published in 1979 (but dated 1939) under DC Comics' FAMOUS FIRST EDITION line.  Originally, the comic had a stiff outer cover identifying it as a reprint.  When the outer cover is removed, it is IDENTICAL to the original book, except that it is OVERSIZED 10" long and 13 1/2 inches tall.  There have been many instances over the years where a non-comic collector has been fooled by one of these reprints.  When I owned a comic book store, I got at least one call per month from someone with an oversized reprint of either Action Comics # 1 (1st Superman), Detective Comics # 27 (1st Batman), Sensation Comics # 1 (1st Wonder Woman title), Whiz Comics # 2 (# 1) (not identical to the original, but not identified as a reprint either), Batman # 1, or Superman # 1.

A genuine Superman # 1 will be the same size as a normal Golden Age comic book, which is slightly smaller than a normal sized magazine

On eBay, a genuine Superman # 1 can often be identified if a picture of the inside front or inside back cover is displayed.  These pages have advertisements for various products from Johnson Smith and Company.  In the original, the address of the company is displayed at the bottom of each ad (Johnson Smith & Co, Dept. 334, Detroit Mich.)  In the reprint, the address of the company is not displayed.

It's worth noting that, while the pages are different sizes, the staples are about the same size in the reprint and in the original.  This means, if you can see the staples, they will generally appear smaller in the reprint than the original.