SAP logical database includes macro implicitly. How?

In transaction SE36 display the logical DB BRF. Then click the pushbutton 'Selections' (or Menu: Goto / Selections or Shift + F1). This will display the include DBBRFSEL, where the selection screen is defined for the logical DB. On the bottom of this include you'll find what you are looking for.

To generalize this for all logical DBs, the selection screen is named DBxxxSEL where xxx is the name of the logical database, and is automatically included in all programs which use this logical database.

In addition to Jozsef's perfect answer I can add that you can utilize table D010INC which holds all include relations between programs and includes.

Thus it's very easy to find such includes programmatically.