window-at-side-p

window-at-side-p is a compiled Lisp function in `window.el'.

(window-at-side-p &optional WINDOW SIDE)

Return t if WINDOW is at SIDE of its containing frame.
WINDOW must be a valid window and defaults to the selected one.
SIDE can be any of the symbols `left', `top', `right' or
`bottom'. The default value nil is handled like `bottom'.