Workspaces do not exist at the X11 level; they're an implementation detail of the window manager, and the only standard concerning them is opendesktop.org's EWMH.  You won't find any X11 or Xorg calls that know anything about them.

xmonad's workspaces exist as part of the StackSet.  At the time a window goes through the ManageHook, it is on the current workspace; any attempt to specify the current workspace before that will fail because it's not yet part of the StackSet (that is, the ManageHook executes as the window is being added to the StackSet, and as a result it will be on the current workspace when the ManageHook executes).  So, while you can easily query the StackSet to see where the window is, there's no real point in doing so.