I'm not sure I understand your question - when a tree is expanded aren't all nodes expanded?
In any case, you can traverse the tree and check for expanded nodes:

lnNodes = thisform.oleTree.Nodes.Count 
for lnCount = 1 to lnNodes
  if thisform.oleTree.Nodes(lnCount).expanded
    messagebox('Node '+transform(lnCount)+' is expanded')
  endif
endfor

>Is there anyone how can help me to understand how you can catch which node is expanded when a treeview control is expanded?