Is there an easy way to detect the version of Sitecore ECM in a Sitecore installation?
The Sitecore solution installed is Sitecore 6.5 rev. 120427 installed.
You can have a look at the package install history in the Core database under /sitecore/system/Packages/Installation history/E-mail Campaign Manager. If you have installed multiple versions then there will be multiple child entries with each revision number.
If you need to get the current running version from code then just get reflect the FileVersionInfo
var assembly = System.Reflection.Assembly.LoadFrom("Sitecore.EmailCampaign.dll");
var fvi = System.Diagnostics.FileVersionInfo.GetVersionInfo(assembly.Location);
string version = fvi.FileVersion;
Slightly 'hack-ish', but you could look to see what packages have already been installed via the Installation Wizard.  When you select a package, it gives you the version number.
Click on the Sitecore button, choose Development Tools, then Installation Wizard.  Choose Browse and then look for the email campaign manager module that was installed.  After you select it, it should show the version number information in the file name.
Caveat: This probably doesn't help if you've installed multiple versions of the ECM over time.