I know this is an old thread, but I found this, and it really helped me find out which item has just been checked/unchecked. Hope it helps someone else!!!
protected void cbYears_SelectedIndexChanged(object sender, EventArgs e)
{
string result = Request.Form["__EVENTTARGET"];
string [] checkedBox = result.Split('$'); ;
int index = int.Parse(checkedBox[checkedBox.Length - 1]);
if (cbYears.Items[index].Selected)
{
}
else
{
}
}