The Replication diagnostics tool is a command-line tool that enables you to report the status of replication on each domain controller. The tool is built into Windows Servers 2008 and Windows Servers 2008 R2 and the information retrieved by Repadmin.exe can help you identify potential problems related to replication within a forest. The detailed views allow you to even identify where and when a change to Active Directory had created problems and hopefully you are still in time to troubleshoot before the problem gets out of control. Another useful feature of Repadmin is that it allows you to create the replication topology and force replication between domain controllers.
To read more about the Repadmin tool go here.