At the bottom of a Confluence page you will see a line like this:
Powered by Atlassian Confluence 2.10.1, the Enterprise Wiki. Bug/feature request - Atlassian news - Contact administrators
In the above it means that you are running Confluence version 2.10.1.
If you do not see that line, you can visit http://<Your Confluence URL>/admin/systeminfo.action and find out the version from there.