Typing X -version should tell you.
It'll look something like this:
X Window System Version 6.8.0
Release Date: 8 September 2004
X Protocol Version 11, Revision 0, Release 6.8
Build Operating System: Linux 2.6.10-rc2 i686 [ELF]
Current Operating System: Linux homer.jtshaw.com 2.6.7-hardened-r17 #2 Tue Dec 14 21:31:00 EST 2004 i686
Build Date: 02 December 2004
Before reporting problems, check http://wiki.X.Org
to make sure that you have the latest version.