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. :-)