gui-selection-exists-p-alist

gui-selection-exists-p-alist is a variable defined in `select.el'.
Its value is

((x . x-selection-exists-p) (nil . ignore))

Documentation:
Whether there is an owner for the given X Selection.
Called with one argument: (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.)