How to find version of XFree86

XFree86 is an implementation of the X Window System. You may need to know which version of XFree86 is running on your computer.

Finding this is easy:

$ X -version

For information about other options:

$ X -help

The X man page does not list any of the options and is not useful for this purpose.

Tried with: X 1.17.1 and Ubuntu 14.04