[Observium] Debugging rsyslog

I have a number of devices on separate VLANs to my Observium server. They are configured to send their logs via syslog to Observium. I have firewall rules in place on our router to allow this traffic between VLANs and I can see the traffic being passed in firewall logs.

However, the log messages never seem to make it into Observium. How can I go about debugging rsyslog to see whether the logs arrive correctly at the server and are correctly processed by rsyslogd?