The following command will useful for 2003 DC health check:
1. dcdiag.exe /v /f:dcdiag_servname.txt
2. dcdiag.exe /test:dns /v /f:dcdiagdns_servername.txt
3. repadmin /showrepl > replication_servername.txt
4. repadmin /replsum /errorsonly > replicationerr_servername.txt
Health check for DCs and/or services assoicated with it.
Let me know DNS health status
This shows all my replication and if it was successful or not. Just be aware that Global Catalogs will have more infor here than a normal domain controller.
repadmin /replsum /errorsonly
This is one takes a long time but will let you know who you are having issues replication with.