Comment 7 for bug 1274740

Yes I think that they always use the same XDG_CURRENT_DESKTOP env variable but DESKTOP_SESSION is different.
I can't test it right now but there you can see some reports by other users: http://ubuntuforums.org/showthread.php?t=2184682&page=19 and http://askubuntu.com/questions/70296/is-there-an-environment-variable-that-is-set-for-unity

(but it's also possible that since GNOME 3.10, they have change some env variables :-) )