Systemd tracks PIDs. This is kind of obvious but I never thought to investigate before today. These two commands have been and I suspect, will be, very useful:
journalctl _PID = 1234
Get info on anything logged by the process in question. Technically, I suppose (?) it shouldn’t tell you anything that systemctl -u whatever.service doesn’t but it helped me get the exact error that caused the service not to work. Thanks, Rahul Khimasia.
systemctl status 1234
How exactly does pulseaudio get started on your desktop? Simply use htop to find the PID and ask systemctl for a status on the it and you will get told what systemd unit is associated with it. session-27.scope, it seems. Didn’t even know there was such a thing as ‘scopes’. Thanks, larsks.