In the Booking Details window, there is a Customer Profile box on the left-hand side.
If the customer has not given your marketing permissions, the profile will reflect that accordingly:
If permissions have been given, you will see a tick to next to Marketing permission. You can then EDIT PREFERENCES if needed.
USEFUL - you can also customise your marketing preferences in your settings to have different options for the customer to opt-in for.
! In the Customers section, you will have an option to then filter by Marketing preferences. See our guide on how-to here.