JSI Tip 9593. FSTOGGLE.EXE freeware can switch a CMD.EXE between windowed and full screen and / or determine the current state.

Download FSTOGGLE.EXE.

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.