How Can I Find the Problems Occurred on MySQL-MariadbDB Systems?

I have built a three-node galera cluster, so it is a Master-Master replication structure. But sometimes the first node goes down and I can’t find the problem causing this. I am only looking at /var/log/syslog files. Where should I look to find the problems? Is there other log files I can look on production environment? Where the database admins looks for db errors?