The lookup functionality is down in the cellrenderer.ListBoxPart object. Here is a snippet showing how you might use it in CurrentCellChanged.
private void gridControl1_CurrentCellChanged(object sender, EventArgs e)
{
GridDropDownGridListControlCellRenderer cr = this.gridControl1.CurrentCell.Renderer as GridDropDownGridListControlCellRenderer;
if(cr != null)
{
string val = cr.ControlText;
if(cr.ListControlPart.FindItem(val, false, 0, true) == -1)
{
Console.WriteLine("Not found");
}
}
}