Hi,
We do not have an event like BeforeActivated that tells which docked control is going to be activated. However, you can use the Enter event of the specific control to get this.
Please refer to this sample and let me know if this helps.
http://www.syncfusion.com/uploads/redirect.aspx?file=DockingFormActivatingEventTest_3f739d74.zip&team=supportRegards,
Nanda