Label someLabel = selectedRow.FindControl("someLabel") as Label;
EDIT:
private static Control FindControlRecursive(Control parent, string id)
{
if (parent.ID== id)
{
return parent;
}
return (from Control ctl in parent.Controls select FindControlRecursive(ctl, id))
.FirstOrDefault(objCtl => objCtl != null);
}
и
Label someLabel = FindControlRecursive(GridView.SelectedRow, "someLabel") as Label;
РЕДАКТИРОВАТЬ 2:
private void imageButton_Click(object sender, EventArgs e)
{
Label someLabel = (sender as Control).Parent.FindControl("someLabel") as Label;
}