ES won't start - where can I check logs?

I recently upgraded ES to 5.6 and it won't start now, and gives no clues as to why. Where are the log files for ES? I have a folder at /var/log/elasticsearch but it is empty. /var/log/messages only shows that it started and stopped, no detail.

Jul 29 10:10:16 locker2 systemd: Started Elasticsearch.
Jul 29 10:10:24 locker2 systemd: elasticsearch.service: main process exited, code=exited, status=1/FAILURE
Jul 29 10:10:24 locker2 systemd: Unit elasticsearch.service entered failed state.
Jul 29 10:10:24 locker2 systemd: elasticsearch.service failed.

How can I get better logging to find out what is wrong? Does ES have a debug mode? Can anyone point me in the right direction?

Thanks!

try checking the output of journalctl.

--Alex