If you're concerned about employees clocking in before they're on site and ready for their shift, you can enable geofencing from your Time Clock settings.
Go to Dashboard > Settings > Timeclock. Switch the toggle to "Yes" and you will see additional options appear which allow you to choose how you would like Sling to identify employees' locations.
Enabling a maximum distance allows you to enter how far from your location (in meters or feet depending on your settings) employees are allowed to clock in. For this setting to work, you must have a street address entered for your location. Learn more here.
Enabling IP address recognition will allow an employee to clock in from their device as long as it is connected to the location's WiFi. In order for this setting to work, you must set your IP address in your location settings. Learn more here.