The Notes C API (and the C++ API) contain functions to identify the unread status of a document. You can also get the unread status of every document in a view using the ?GetViewEntries URL command, which is documented in the Domino Designer help.
I don't know of any way to retrieve the unread status of a document through the Java/CORBA interface (on which DCO is based).
This was first published in October 2003