The only reliable way I found is to check the log.
/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
Example output with an error:
Jan 9 19:10:57 r530a cron: Error: bad minute; while reading /etc/crontab
Jan 9 19:10:57 r530a cron: (*system*) ERROR (Syntax error, this crontab file will be ignored)
Jan 9 19:19:01 r530a cron: (*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)