LocationManager locationManager = (LocationManager) getActivity().getSystemService(LOCATION_SERVICE);
        if ( !locationManager.isProviderEnabled( LocationManager.GPS_PROVIDER ) ) {
            AlertNoGps();
        }


    private void AlertNoGps() {
    AlertDialog.Builder builder = new AlertDialog.Builder(getActivity());
    builder.setMessage("El sistema GPS esta desactivado, ¿Desea activarlo?")
            .setCancelable(false)
            .setPositiveButton("Si", new DialogInterface.OnClickListener() {
                public void onClick(@SuppressWarnings("unused") final DialogInterface dialog, @SuppressWarnings("unused") final int id) {
                    startActivity(new Intent(android.provider.Settings.ACTION_LOCATION_SOURCE_SETTINGS));
                }
            })
            .setNegativeButton("No", new DialogInterface.OnClickListener() {
                public void onClick(final DialogInterface dialog, @SuppressWarnings("unused") final int id) {
                    dialog.cancel();
                }
            });
    alert = builder.create();
    alert.show();
}