In that case you could fire a custom event every time you change the visibility, as far as I know you should be able to figure out if widget X is visible or not by simply asking it ( component.isVisible ).
The only thing is that you will have to ensure that you fire this custom event everywhere in your code where you influence the visibility of a component... To overcome that I would create a custom component and overwrite the (setVisible method, to automatically trigger the event for you) of course as mentioned by Jeff you do need to really make sure that you are not messing it all up with inline styles, css or other methods of messing about with the visibility of your component.
All in all it is not an easy one and it will mean you have to be 100% sure that you never ever set visibility in any other way then problematically which is depending on your project not that hard or near impossible...
Regards,
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