If you are using cron, normally an email will be sent to the owner
of the script (eg root if the command is in the root crontab) if
there is any output from the script.
In my experience s3cmd sync usually generate output even if
there isn't anything to update so you should be getting emails
now.
Cron sends the output of all commands to the email account of the user that
runs the job. This may be a local email drop on the box if you haven't set
up email for your vm.
You can change the address that cron sends the output to by setting the
MAILTO environment variable in the top of your cronta
