Perform a directory dump of all users without a uSNChanged attribute value. Make a simple change to the properties of the user object, and this will populate the uSNChanged attribute with a value.
Refer to Knowledge Article 15721 - "How can the LDIFDE utility be used to troubleshoot migration issues?"
In order to find any objects in Active Directory which do not have the uSNChanged attribute populated, the following LDAP query can be used:
The ! operator in conjunction with the wildcard operator will look for objects where that attribute is not set to anything.
The uSNChanged attribute is used by active directory for Directory Replication purposes to determine which server has the last updated copy of the object.
Additional information about LDAP query basics can be found on Microsoft TechNet: http://technet.microsoft.com/en-us/library/aa996205(EXCHG.65).aspx