Right now is that there is no way to tell which slots are overbooked unless you go into every time slot to check (the can be dozes or more per day) or export all the bookings. Exporting to check overbooking is really not ideal as people continue to book and the list keeps updating in real time. Waiting until the day of the event to pull the final roster (export) to check overbooking will be too late for merchants to reach out to customers to adjust their time slots.