Query For All Constraints in a SQL Database Diagram
In SSMS, if you expand the tree of tables for your database, then expand the Columns folder, the icon next to a column will be a silver/gray key if it's part of a foreign key relationship, but unfortunately it won't show you the column to which column it's mapped.

You could also create a new diagram in SSMS by right-clicking the "Database Diagrams" folder underneath your database in the tree and choosing "New Database Diagram." You will get lines between the tables where foreign keys exists.

You could also use third-party tools to reverse engineer a diagram from your DB schema, like Microsoft Visio or Sparx Enterprise Architect.

I'm not sure SSMS Express supports these things, so you might be out of luck with anything fancy/visual.