In the Booking Details window there is a Customer Profile box on the left hand side.
If the "Marketing" box is ticked this means the customer signed up to receive newsletters from you when making their booking/enquiry. If the box is unticked then the customer didn't want to receive newsletters from you. If you do manage to sign up a customer when you speak to them, simply click this box.
In the Customer tab, you will see a "Marketing" column. You can use your filter tool to filter by Marketing permission, selecting 'yes' or 'no'.