I don't think there is anything in the API that will help you. There isn't a property (or event) for expanded/collapsed.
If you're looking to do something within the chart, you could use the Dimensionality() function. If you add Dimensionality() as an expression in your chart, it should equal one when your dimensions are all collapsed. When you open the first one, it should go to 2 and so on. This would allow you to vary the expression depending on the expanded/collapsed state of that record.
I don't think there is way to access this information outside of the chart. Maybe if you could describe what you're trying to do in more detail, somebody could come up with something.