Group Layer Visibility

Does anybody know of a way to track the visible state of a group layer? It seems as though the visibleLayers array does not include the group layers and defaultVisibility isn't helpful either.

What I am trying to do is develop a widget that will allow the user to save the current map state, sort of like a bookmark, but to include layer visibility and basemap visibility. I've got what I need EXCEPT I have no way of knowing when a user turns on/off a group.

Any help is appreciated!

Thanks,
Angie