To check which version of Ubuntu you are running

To check which version of Ubuntu you are running, click System -> About Ubuntu. The first line of the page that loads will tell you the version number, for example Ubuntu 10.04 LTS.

Check the list of Ubuntu versions to see how old your version is and whether an upgrade is available.

Checking with the command line

To check your Ubuntu version using the command line:

   1.Open a Terminal (Applications -> Accessories -> Terminal)
   2.Type lsb_release -a and press Enter.(or else you can type this commands also  in terminal #> sudo nano /etc/apt/sources.list  #> lsb_release -a).

  3.The Ubuntu version will be displayed on the screen.

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For GUI mode :

To make up for not blogging over the past few weeks I’ve got a few things up my sleeve here. I definitely want to get back into my old habit so I’ll just dive right in.
This tutorial will outline a few methods of finding your installed Ubuntu version or kernel version. These can be useful if you ever need to troubleshoot a problem or need more information for a bug submission.
The first method you can use is a GUI method to see what version you have installed. Personally I think it could be made a bit more prominent, but that isn’t my call. To find the version using the GUI method simply do the following:

System > About Ubuntu

Thanks to:

http://ubuntu-tutorials.com/2007/01/27/how-to-find-your-ubuntu-or-kernel-version/

https://help.ubuntu.com/community/CheckingYourUbuntuVersion