The easiest way is to change your calendar preferences to show an icon for new patients. This displays a trophy icon next to the name of any patients who have never attended an appointment with the practice and allows you to easily see whether you need to send a patient intake form, review a patient's completed intaked form or collect other information from a patient.
To protect patient privacy, whether or not the new patient icon is displayed may depend on the user's access level. For example, if a Personal Access practitioner cannot see that another practitioner at your practice has previously seen the patient, the icon will be displayed for the Personal Access user as though the patient is new. However, a Full Access user who can see appointments with the previous practitioner will not be shown the icon.
Alternatively, you can run a report to generate a list of patients that fit certain criteria - for example, an appointment report with the Report Type set to "Attendance" and filtered for patients with only 1 appointment total may suit your purposes. See Useful Campaigns for Your Practice for some ideas on how to capture the data you want and any messaging you may wish to send.