Before I upgrade a domain I usually check for problems and issues. I use the following steps to give me an idea of the status :
dcdiag /v
(Check DC en services)
netdiag /v
Check networking components
netsh dhcp show server
Show DHCP status
repadmin /showreps
Show all replication between sites
repadmin /replsum /errorsonly
Show replication issues
When you perform the steps mentioned and you didn’t ran into several errors than you have probably nothing to worry about. 🙂