Is there a way to validate /etc/crontab’s format?

The only reliable way I found is to check the log.

cron checks /etc/crontab every minute, and logs a message indicating that it has reloaded it, or that it found an error.

So after editing, run this:

sleep 60; grep crontab /var/log/syslog | tail

Or, to not wait a full minute, but only until the next minute + 5 seconds:

sleep $(( 60 - $(date +%S) + 5 )) && grep cron /var/log/syslog | tail

Example output with an error:

Jan  9 19:10:57 r530a cron[107258]: Error: bad minute; while reading /etc/crontab
Jan  9 19:10:57 r530a cron[107258]: (*system*) ERROR (Syntax error, this crontab file will be ignored)

Good output:

Jan  9 19:19:01 r530a cron[107258]: (*system*) RELOAD (/etc/crontab)

That's on Debian 8. On other systems, cron might log to a different file.

(I thought I could avoid hunting for the right log file by using systemd's journalctl -u cron, but that didn't show me these log entries, and actually seems to have stopped logging cron events 2 days ago for some reason)

Another more recent solution is the python script chkcrontab