As a vintage Curious George collector for many years, I am often asked how to spot a true First Edition version of the original book versus a fake one.  The easy answer is this:  If you see red, it's real! 

The First Edition is available in two different cover versions, hardback and library binding.  Both versions have RED covers.  I own both versions. 

The inside cover page of either version shows a copyright date of 1941 only ("Copyright, 1941, by H. A. Rey") and the words "Lithographed in the U.S.A. by William C. D. Glaser, New York" (hardback version) or "Lithographed in the U.S.A." (library binding).  The library binding is slightly larger in size than the hardback version.

Margret Rey, wife of H. A. Rey, renewed the copyrights of all 7 of the original Curious George books before her death in 1996.  Therefore, anything other than a true First Edition will have an additional copyright date after 1941 and will include Margret Rey's name.  Most importantly, the cover on all subsequent versions is yellow.