By default, systemd services will log their output to /var/log/messages, and you can view these messages with journalctl commands.
To tail the logs for a specific service you are running, you can simply do the following.
journalctl -u service-name -f
Just remove the -f if you want to view the log in general.
You can also just lazily use -f without the -u and service name if you want to tail recent system logs, which is often useful.