Logs for OpenVPN are located in the GUI under Status > System Logs on the OpenVPN tab.
These logs include output from the OpenVPN daemon(s) in use, both clients and servers. Log messages include entries for successful connections as well as failures and errors.
If there are no log entries for a server after the process starts, traffic likely is not reaching the OpenVPN daemon. Check the WAN-side firewall rules and the address/port used by the client.