x-server-version

x-server-version is a built-in function in `C source code'.

(x-server-version &optional TERMINAL)

Return the version numbers of the X server of display TERMINAL.
The value is a list of three integers: the major and minor
version numbers of the X Protocol in use, and the distributor-specific release
number. See also the function `x-server-vendor'.

The optional argument TERMINAL specifies which display to ask about.
TERMINAL should be a terminal object, a frame or a display name (a string).
If omitted or nil, that stands for the selected frame's display.