x-selection-owner-p

x-selection-owner-p is a built-in function in `C source code'.

(x-selection-owner-p &optional SELECTION TERMINAL)

Whether the current Emacs process owns the given X Selection.
The arg should be the name of the selection in question, typically one of
the symbols `PRIMARY', `SECONDARY', or `CLIPBOARD'.
(Those are literal upper-case symbol names, since that's what X expects.)
For convenience, the symbol nil is the same as `PRIMARY',
and t is the same as `SECONDARY'.

TERMINAL should be a terminal object or a frame specifying the X
server to query. If omitted or nil, that stands for the selected
frame's display, or the first available X display.

On Nextstep, TERMINAL is unused.