How can I tell which release my org is running?

On the Home tab, on the top right of the screen is a link to Discover Winter '14. This indicates your release.

Or if you like to see it in xml, goto /services/data/ on your instance.

https://[instance].salesforce.com/services/data/

works with a lightning url too. https://[domain].lightning.force.com/services/data

You can see in more detail what release and patch level your instance is on in the Releases section of the known issues page.

For example this page, which lists fixes for Summer 14, https://success.salesforce.com/issues_releases_view?release=190000000, also shows the patch level of each instance - at the time of writing "Spring '14 Patch 20.2". This can be useful if you see discrepancies in behaviour between orgs on different instances.