You have a list of incredibly powerful reports in your Report List, via Reporting—one of which is called the Client Marketing Report.
In the Client Marketing Report you'll find numerous rows with very useful information and figures, one row in particular looks at your Manual Reminder Clients—and who are these people you ask? These are your clients who the system has detected as needing to be manually reminded of their appointments. The way the system knows is because they either don't have a mobile number in their personal details, or they are not enabled for SMS Reminders.
Let's take a look.
Go to Reporting > Report List > Client Marketing Report > change the date range > click Show Details for the Manual Reminder Clients.
You will have your list of clients who have appointments between the dates you choose and who need to be reminded of their appointment manually (i.e. you'll need to give them a call to remind them of their appointments).
NOTE: Keep in mind though that if you have your email reminders enabled, and any of these clients have an email address and have been turned on for email reminders, then they will be reminded that way—via email. Therefore, this report is only looking at clients who are not being reminded about their appointment via the automatic SMS.
Perhaps these clients have requested not to receive SMS reminders, or maybe they have been overlooked and you've forgotten to turn them on for SMS reminders in their personal details or you've forgotten to put their mobile number in the Mobile section of their personal details. Either way, it's a good idea to check each client in case they shouldn't be on this list and they should, in fact, be receiving SMS reminders for their appointments—this will minimise no shows for you.