Jeffrey,

you could test the LastModificationTime propery (actually a DateTime) of each ContactItem in the collection.If you haven't already done so, you need to study the Outlook Object Model (actually quite clear an easy to understand, for a change). Here's a URL to start you off :
http://msdn.microsoft.com/library/default.asp?URL=/library/officedev/off2000/olobjcontactitem.htm

Hope this helps.

Best wishes,
Neil