You can check the availability checker. Existing bookings are shown in purple, but that does not mean that everything on the site is booked. There could still be availability for the activity / camping pitch that you would like to book. You can hover over the existing booking on the calendar below to see what is already booked.
(Please note that the calendar is refreshed hourly, so if you make a booking and return to the calendar, it will not be shown immediately)