Quick guide on getting a GUID from the URL in CRM 2013: (they look as though they have removed them in CRM 2013, but they are just hidden!)
Open the form you want the GUID of and click the ‘pop out’ icon found in the top right corner:
This will open your record in what looks like a new window, but now if you look at the URL:
You can see the GUID nested nicely in between curly braces. If you just want a sanity check, click the ’email a link’ icon to get the reference in a link, and you can cross check the GUID against the GUID in the email a link, starting after the small ‘b’ ending at the one before the ‘%’.:
You can see they both match!