How can I know my app is working in background?

I have written this program, it is work when the app is displaying on screen

If the app in background, it stop to print the latitude, when I resume the app, it will start to print again.

I have enabled background modes in xcode, and also checked Location updates, why my app still not running in background?

If the app is running, just the print function does not work in background, how can I know the app is running?

class ViewController: UIViewController,
    CLLocationManagerDelegate {

    var locationManager: CLLocationManager = CLLocationManager()
    var startLocation: CLLocation!

    func locationManager(_ manager: CLLocationManager,
                         didUpdateLocations locations: [CLLocation])
    {
        let latestLocation: CLLocation = locations[locations.count - 1]
        print(latestLocation.coordinate.latitude)    
    }

    override func viewDidLoad() {
        locationManager.desiredAccuracy = kCLLocationAccuracyBest
        locationManager.delegate = self
        locationManager.requestWhenInUseAuthorization()
        locationManager.startUpdatingLocation()
        startLocation = nil

    }
}

If your app uses location in the background ).you have to set allowsBackgroundLocationUpdates to YES in addition to setting the background mode capability in Info.plist. Otherwise location updates are only delivered in foreground.

if CLLocationManager is first called startUpdatingLocation method, and in the projectname-Info.plist file is added Required Background Modes -> App registers for location updates.