When you type fstoggle /?, you receive:

FSTOGGLE.EXE usage:                                      fstoggle        toggles the display mode                                      fstoggle 1      switch to fullscreen mode                                      fstoggle 0      switch to windowed mode                                      fstoggle ?      print the display mode (0 or 1)                              Any other syntax shows this message.                              In any case, FSTOGGLE returns 0 or 1, indicating                              the display mode as it exits.