How to confirm that drush-based cron is being called from crontab? I don’t know whether this is a docksal-specific question or a general cron one, but likely the latter.

I’ve set up a crontab for docksal (see https://blog.docksal.io/running-scheduled-events-with-docksal-43fc69d7b29b): 

0 */3 * * * drush --root="/var/www/docroot" core-cron

My question is how tell if it’s actually working. I don’t see anything logged in at /admin/reports/dblog. I also tried enabling the syslog module, but I’m not seeing a syslog file at /var/log as described, using fin bash and cding to that directory.

I don't whether I should expect to see something, but would prefer I did. Otherwise it's like trying to check if the refrigerator light has gone off when you've closed the door.
It sounds like you've answered your own question. Viewing the admin DB log reports or running something like fin drush wd-show --type=cron should show cron execution entries. If you're not seeing those logs, it most likely means the crontab config isn't working. 

You should check drush --root="/var/www/docroot" core-cron works correctly when manually executed in the container where crontab is configured (e.g. perhaps the crontab user doesn't have drush available in its path or drush can't find the DB config) &amp; the /var/log/ dir (it will depend on the Linux distro the Docksal image is based on) for cron logs to confirm that its firing.