The value of the highestCommittedUSN attribute on rootDSE is equal the the highest USNChanged value on that particular domain controller. You can use it to determine what objects were changed recently on that DC. For an overall view of recently changed objects and how they have been replicated, you can perform these queries on each domain controller and compare the results.
Here is a great technet blog post on tracking recent changes in AD:
http://blogs.technet.com/b/askds/archive/2009/03/18/how-do-i-find-out-what-changes-are-going-on-in-my-active-directory.aspx
