The title bar of the CMS edit mode should give you the version info of the site, at least the CMS version. Otherwise you could always check the dll:s in the bin folder or the assembly-redirects in web.config.

Thanks Linus

In more recent versions, Edit mode only shows: EPiServer CMS - Edit, but Admin mode title does include the version.

For these cases we usually giving some authorized endpoint (somestimes it does matter what version of your own app is installed):

public class VersionController : Controller
{
    [Authorize(Roles = "CmsAdmins")]
    public ContentResult Index(string assemblyName)
    {
        var fvi = FileVersionInfo.GetVersionInfo(Assembly.GetExecutingAssembly().Location);
        var version = fvi.FileVersion;

        return new ContentResult { Content = "Current version: " + version };
    }
}

The method used to display title (mentioned by Marija) is available as public, static:

 EPiServer.UI.Edit.MasterPages.Frameworks.Framework.CreatePageTitle((string) null, LocalizationService.Current)

and could be reused in your VersionController:

public class VersionController : Controller
{
    [Authorize(Roles = "CmsAdmins")]
    public ContentResult Index(string assemblyName)
    {
        return new ContentResult { Content = "Current version: " + Framework.CreatePageTitle(null, LocalizationService.Current) };
    }
}