Relationships and RI are defined ONLY in the back end database. It doesn't matter whether the back end is DB2, Oracle, SQL Server, or Jet. It is the database engine that enforces RI and so it needs to be defined where the data lives.
You can view relationships in the fe by opening the relationships window. If the relationships are not immediately visible, you will see them after you add the tables to the window. As you add two related tables, Access automatically draws the relationship lines.