@Override
public void onResume() {
    super.onResume();
    //Make your toast here
}

class MainAvtivity extends Activity {
    static int currentFrag = -1;
    ....
}

@Override
public void onResume() {
    super.onResume();
    if(MainActivity.currentFrag == 3)
        //Make toast here
    MainActivity.currentFrag = <current_fragment_number>;
}