- Open the page in compatibility mode in a newer IE. This should give you the correct file, and line number of the error.
SharePoint 2013, have done this when checking if window.JSON
ect is defined if it's not it will throw a error. See http://stackoverflow.com/q/23404556/171087 for more information.
- Open Google Chrome, in the Source tab search for the error. This might just find the file in question, and the function that throws the error-