How to detect the Primary Key and Foreign Keys in the universe?

Hi All

       Could anyone help out in knowing the various ways to detect the primary key and foreign keys across the tables being used in the Universe.

I have to modifying the existing set of joins being used in the universes. One way which i figured out from one of the Youtube Videos,is to search for the duplicate values of the specific (assumed fields for primary and foreign Keys) in the "Column values" option which gets popped up upon selecting the respective fields and right clicking the table.

                                      But question is what if there are no duplicate values(then how do we differentiate between the primary keys and the foreign keys used in the tables?