It's best to record if clients have arrived or not when they visit the clinic. In Better Clinics, we implemented a way to add the arrival status of the client on the appointment popup:
To check in a client click on their scheduled appointment.
On the appointment pop up click the 'Check In?' box under the client's initials.
When clicked once, the arrival status of the client will change from 'Check In?' to a green 'Arrived' status.
When the Avatar is clicked again, the status will change to red and update to 'No Show'. The statuses are: Default (no status - Grey), Arrived (green tick), No Show (red tick).
Each time the Avatar is clicked, the status will change. When it gets to red and clicked again, the colour will change to Grey, and the loop will continue again.