There is no specific way that you can achieve that in Anaplan. Couple of options that you have already tried are having a code for line item and Checking Modules ->Line items glossary. One more option is taking the export of all line items and filter it outside Anaplan
You can update the module name in the Notes of line items ( which is normally used to identify the purpose and context of any object in a model). This way when you search for line items in the module tab, you can scroll to Notes section to check the module name.