The ER diagram picture draws the relationship with a line that has two two icons on each end - a key on one side (the “parent” table) and a chain link on the other side (the “child” table.) The icons are placed on the same line as the column(s) that forms the relationship.
Sometimes that can be hard to see (especially with several columns on the relationship.) Then you can double-click on either of the table names, which will open an Object Details window (by default this is a docked window at the bottom of your screen.) You may need to resize the Object Details window by dragging its top size bar up.
In the object details window, click on the constraints tab (make sure that the object details window is tall enough to show the list of constraints). Select the constraint and you will see the participating columns in the bottom of the object details window.
If this is not clear let me know and I can send you a screenshot.
I agree, it would be nice to show this information in a tooltip or through some shortcut menu. I will discuss this possibility with our development team.