Accordion activate event. How to detect if a tab is opening or closing

Accordion activate event. How to detect if a tab is opening or closing

I'm using a only one tab collapsible accordion, and I would like to detect the "open" operation and to distinguise it from the "close" operation, in order to load the tab via an ajax query during the opening.
Any suggestion in order to detect the open and close operations (in the "activate" event, I suppose)?