Assuming you have a search that updates your lookup file, the easiest way would be to add another field
last_update to your lookup entries. This way you can always tell when the respective entry has been last updated.
The lookup file has the "Last_Updated" field, but it has different timestamps associated with each record. Also, the lookup file is uploaded by the end-users.
You can still load the lookup file and check for the latest "last_updated" field.
As far as I know there is no built-in possibility to check for a file update.