Function: window-combined-p

Return non-nil if WINDOW has siblings in a given direction.
WINDOW must be a valid window and defaults to the selected one.

HORIZONTAL determines a direction for the window combination.
If HORIZONTAL is omitted or nil, return non-nil if WINDOW is part
of a vertical window combination.
If HORIZONTAL is non-nil, return non-nil if WINDOW is part of a
horizontal window combination.