You can mark a patient checked-in for an appointment. First select the appointment in the Scheduler, then right click on it and select Checked-In.
Use the spin buttons to change the Arrival Time, if necessary, and click Ok.
Once checked-in, a small green check mark will appear in front of the name in the appointment slot.
To view a list of patients who have been marked as checked-in for the current date, select Checked-In from the View menu.