How to determine the right REAL system user which record in this entity belongs to?
eg: task created by SYSTEM user, but I need to know the real user. (record owner is not a way when owning user can be changed when he leaves the company)
This depends on how that task is being really created. If I remember correctly, SYSTEM user is captured when entity records are created by portal user, Workflow/Plugin running on context, and some other scenarios.
Sometimes, ETL integration also does that, in those cases probably you have to check the createdonbehalfby column as well for accuracy.