A customized Booking List will work well for seeing which Bookings do & don't have referral sources. You'll just need to add the 'referral source' column.
Click on Bookings in the left-hand sidebar
Click List (if you were on the Pipeline)
(optional) Add any necessary Filters to narrow down your search
Add the Referral Source Column (along with any others you may like)
Sort the columns by clicking the arrows next to the Referral Source header
(optional) Save View
For more information on customizing & saving Views, see our article on Custom Views & Reports