fragmentManager.addOnBackStackChangedListener(new FragmentManager.OnBackStackChangedListener() {
        @Override
        public void onBackStackChanged() {
            // If the stack decreases it means I clicked the back button
            if( fragmentManager.getBackStackEntryCount() <= count){
                //check your position based on selected fragment and set it accordingly.
                navigation.getMenu().getItem(your_pos).setChecked(true);
        }
    }
});