By default `stdout` and `stderr` of a systemd unit are sent to **syslog**. On my machine(Kali distro) it is stored in `/var/log/syslog`. So to see tor unit output run

    cat /var/log/syslog | grep tor -i

# Edit
In case someone needs to monitor tor unit i found a better way. Adding `-f` or `--follow` flag to `journalctl` makes it to continuously print new entries as they are appended to the journal. You can then pipe it to a parser and react based on it.

In one of your termianl tabs run

    sudo journalctl -f -u tor@default | grep bootstrapped -i

in another tab do

    sudo systemctl restart tor

You will see the results are appended to the first terminal as they are appended to the systemd journal