Whenever you get a booking, you can then go into the booking and view the "Website and source" tab to see the Booking source. Here you can also add a source from the drop down menu if it wasn't selected previously.

If you are creating an admin booking, you can select a Source before you click Save on the booking.

You might be reminded with  a messageby the system that you have to add a booking source, if this was made a compulsory entry.