I found the answer. here is what i did

 private void GetSelectedNodes(XamDataTreeNodesCollection nodes)
        {
            foreach (XamDataTreeNode node in nodes)
            {
                if (node.IsChecked == true)
                    _selectedNodes.Add(node);

                GetSelectedNodes(node.Nodes);
            }
        }

then i just call the function

 GetSelectedNodes(pgaDataTree.Nodes);