With Active Directory, what is a good way to monitor replication?

I have multiple sites and multiple locations, so ideally both replication between sites and within sites would be monitored. I'm not really sure if each DC needs to be monitored, each NTDS connection, or each DC * Each NTDS connection.

For the purposes of fitting into a standard alerting methodology, perfmon counters that would allow me to alert if replication was behind X minutes seems like it might be ideal.