I'd have thought if the French had found Guynemer's body you'd have heard the cheer around the world. It seems to be generally accepted that Guynemer's body and his Spad were destroyed by an artillery bombardment south of Poelcapelle.
As for indentification, I'd say that Guynemer's relatives would have to be traced and a DNA match achieved (in a similar vein, Mannock's family were approached in the early 1990s to get their help in identifying the pilot identified by Jim Dudgeon as being
Mick Mannock's remains. The family declined to become involved). Failing that and since he would most likely still be close to his aeroplane, anything that identified the Spad as being Guynemer's would have to be found.