How can I determine if a process is a system process?

I'm reading the /proc directory (or pseudo-fs) to find all processes. I'm getting the information I need from /proc/[pid]/status but there's something else I need. Is there any way to figure out which processes are critical to system? for example using parent-pid or UID of the process?

By system process, I mean processes that would otherwise exist on a fresh installation of the OS, and before installing any application or services. This might not mean kernel threads, or system processes at all, but to sum it up, I mean processes, that their termination, would disrupt the fundamental structure of the system.

PS. I'm working on an android app, but since this part is done using pure Linux file system I asked it here and I don't suppose that there would be any different.