Where to find CRON logs?

Start with journalctl --unit crond -n all.

But in general, it is a mess. The correct solution seems to be piping command output to the system journal within the cron script. Don’t forget to redirect stderr. Example that does both:

systemd-cat -t example ./do-the-work.sh

(then journalctl -t example)

Health monitoring (dead man’s switch)