If the user has turned off location services for your app the first time, you will know in your application every time you try to get their location. You can check if the user has given you permission to read their location or not. In case they haven't, they will be prompted again to enable location services for your app.
Quoting from Apple's documentation (found here)
it is recommended that you always call the locationServicesEnabled class method of CLLocationManager before attempting to start either the standard or significant-change location services. (In iOS 3.x and earlier, check the value of the locationServicesEnabled property instead.) If this class method returns YES, you can start location services as planned. If it returns NO and you attempt to start location services anyway, the system prompts the user to confirm whether location services should be reenabled. Given that location services are very likely to be disabled on purpose, the user might not welcome this prompt.