"foreign key" is a term out of database_management I think. It's not so relevant in QlikView.
What you have to care about is only, when you hover over the key_field of a table in the Table_Viewer in QlikView, whether it reads "primary key" or "perfect key" - both of those mean the key is sufficient to uniquely identify a record - or just "key" - that means there are duplicates on this field.
