The AppBarLayout has a Switch to be notified of changes of offset.
I believe it is possible to be notified when it is completely collapsed or expanded as follows:
AppBarLayout.OnOffsetChangedListener  listener = new AppBarLayout.OnOffsetChangedListener() {
    @Override
    public void onOffsetChanged(AppBarLayout appBarLayout, int verticalOffset) {
        if(collapsingToolbar.getHeight() + verticalOffset < 2 * ViewCompat.getMinimumHeight(collapsingToolbar)) {
            // CollapsingToolbar esta colapsado
        } else {
            // CollapsingToolbar esta expandindo
        }
    }
};
appBar.addOnOffsetChangedListener(listener);
Source: https://stackoverflow.com/questions/31595741/show-view-when-toolbar-collapses