1. Go to the list settings page.
2. Right click on "Title, description and navigation" and copy the shortcut.
3. Ctrl-V into notepad.
4. Everything after "List=" in the string is your URL Encoded GUID for the list.
This can be decoded with Microsoft.SharePoint.Utilities.SPEncode.UrlDecodeAsUrl();
or as needed by the wonderful utility at meyerweb.com