/run/systemd/system/
is allegedly the most reliable way to tell that debian is running systemd.
readlink /sbin/init
service --status-all
(SysVinit)
ls /etc/init.d
(SysVinit)
systemctl list-units --type=service
systemctl list-units --type=service --all
(show inactive services as well) /etc/inetd.conf