How to determine selected index change for drop down box, before the selectedIndexChanged event is executed.

When a user changes a drop down box selection, I get post back to my C# code, and the event "selectedindexchanged" is executed in my C# code:
protected void ddlNames_SelectedIndexChanged(object sender, EventArgs e) {
}
This works great, however sometimes, I need to determine if the drop down box value has changed before the _Sel ...