systemd is the parent process to all child processes running on QubesOS.
Is it possible to login to a Xen/QubesOS systemd instance remotely?
Can SSH, D-BUS, Saltstack be used to to this?
How could a user determine if there was indeed a remote connection
to their QubesOS systemd instance?
@janglingquo_575, could you please explain in a bit more detail what you mean when you say the term “systemd instance”…?
It’s definitely possible to log into a qube remotely if you allow incoming connections to it. It’s the same as a regular machine. It’s depends on what software is running inside of it, and what access it has to/from the outside world.
Yes, if you allow the incoming/outgoing connections. In fact, the SaltStack modules are quite comprehensive 
Exactly the same as any other bare metal computer or VM. Check the logs of whatever software you’re using for the remote connection(s).
I have provided very vague answers because I’m not quite sure what you mean when you say “systemd instance”. I’ll provide updated answers if necessary once you define it a bit more clearly, but I think those answers above already answer your questions
The systemd instance runs inside dom0, which isn’t networked.
systemd is a complex piece of software, that’s my only concern.
But since it is isolated inside of dom0, which isn’t networked,
the idea is that it can’t be remotely accessed because you can’t network
dom0.
I had a feeling that was what you meant.
It can be accessed if you really want, but you can get all the equivalent information via other means that don’t require direct dom0 access anyway.
And it usually doesn’t play nice with other software that performs the same functions as systemd components, at least without a log of troubleshooting.