To achieve your requirement, you can use the RowIsExpandedChanged event of RadTreeListView. Then get the row from the event arguments and check its IsExpanded property. Here is an example in code:
private void RadTreeListView_RowIsExpandedChanged(object sender, Telerik.Windows.Controls.GridView.RowEventArgs e)
var row = e.Row as Telerik.Windows.Controls.TreeListView.TreeListViewRow;
var isExpanded = row.IsExpanded;
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