How can I check the installed AMD Catalyst driver version?

dmesg | grep fglrx | grep module will output something including a line with something like module loaded - fglrx 9.0.2 [Sep 20 2012]. The 9.0.2 part is the Catalyst version.

Note: in order to compare the fglrx package's version to AMD's version numbers, see How does the fglrx package's version relate to AMD's version numbers, like Catalyst 12.3?.

If you installed fglrx from the repositories or via a deb you can get the version from apt-cache:

apt-cache policy fglrx

this will give

fglrx:
  Installed: 2:9.012-0ubuntu1
  Candidate: 2:9.012-0ubuntu1
  Version table:
 *** 2:9.012-0ubuntu1 0
        100 /var/lib/dpkg/status
     2:8.960-0ubuntu1.1 0
       500 http://nl.archive.ubuntu.com/ubuntu/ precise-updates/restricted amd64 Packages
     2:8.960-0ubuntu1 0
        500 http://nl.archive.ubuntu.com/ubuntu/ precise/restricted amd64 Packages

As you can see I installed version 9.012 (from a deb I made from the amd site) and the version in the repositories is 8.960.

Or you could do Jeff's solution which also works if you didn't install using apt (not installing with apt is not recommended!).

on my system AMD Catalyst Linux 13.1 driver Revision Number: 13.1 (9.012) shows for your command a different fglrx version then fglrxinfo. just FYI.

t@t-ubuntu:~$ dmesg | grep fglrx | grep module
[    3.768319] fglrx: module license 'Proprietary. (C) 2002 - ATI Technologies, Starnberg, GERMANY' taints kernel.
[    3.839422] [fglrx] module loaded - fglrx 9.1.11 [Dec 19 2012] with 1 minors

t@t-ubuntu:~$ fglrxinfo
display: :0  screen: 0
OpenGL vendor string: Advanced Micro Devices, Inc.
OpenGL renderer string: ATI Mobility Radeon HD 5800 Series 
OpenGL version string: 4.2.12002 Compatibility Profile Context 9.012