How to compare Active Directory on 2 Domain Controllers

Solution 1:

It sounds like your plan is to determine what DC has the latest changes, and then make them on another DC?

No no no no no.

This will backfire. AD doesn't replicate by what changes are consistent with each other, it replicates by latest serial number. What you need to do is fix replication. Depending on your version of Windows (you didn't tell us; it would be helpful), you can use REPLMON or REPADMIN to determine what is failing, and probably why. Edit- cheekaleak is correct: DCDIAG is also useful for finding replication and other errors in your DCs.

Solution 2:

Run dcdiag on the server that isn't replicating changes made on it, and check its File Replication Services log for errors. That should help point you in the direction of how to fix replication.