How to determine unity or gnome-shell or something else is running in C

[YunQiang Su]

 I am trying to make a program has different tray in different environment:
   when indicator is supported, indicator plug-in is used,
   when in gnome-shell or gnome-panel, notification area written by  gtk
   when KDE, use the way KDE prefer,

 Then the problem is that how I can determine which are using dynamic?
 unity, gnome-shell or something else ?

-- 
YunQiang Su