Determining current section of TableView

I'm trying to figure out, as the user is scrolling through a TableView, what section is currently visible on the screen.

I can see in the scroll event what the contentOffset is but mine are variable heights. On top of that there are additional rows added in to the mix from time to time. Calculating that every scroll event wouldn't work.

Is there an event, such as 'sectionChanged' or a property on the TableView that I'm missing that would allow me to do this?

Thanks,

Tom