public void onProviderEnabled(String provider);
public void onProviderDisabled(String provider);

<action android:name="android.location.PROVIDERS_CHANGED" />

<receiver android:name=".gps.GpsLocationReceiver">
        <intent-filter>
            <action android:name="android.location.PROVIDERS_CHANGED" />
            <category android:name="android.intent.category.DEFAULT" />
        </intent-filter>
    </receiver>

public class GpsLocationReceiver extends BroadcastReceiver implements LocationListener
...

@Override
public void onReceive(Context context, Intent intent)
{
        if (intent.getAction().matches("android.location.PROVIDERS_CHANGED"))
        {
            // react on GPS provider change action
        }
}

try {
        locationManager = (LocationManager) getSystemService(LOCATION_SERVICE);

        if (locationManager.isProviderEnabled(LocationManager.GPS_PROVIDER)) {

            Log.i("About GPS", "GPS is Enabled in your devide");
        } else {
            //showAlert
        }