Re-posting here from the other thread, I would propose a simpler argument to show the existence of an objective reality outside the subjective realm, with less leaps of faith.
I would propose the following demarcation: in your subjective experience, there are things that resist and act as obstacles to your will, or constraints on your subjective experience.
For example, no matter how hard you try, if you jump up, gravity will bring you down. Furthermore, that is the common experience of everybody else that you perceive in your experience.
Those things that resist or act as constraints on your subjective experience, we could call "objective". They appear to your subjective consciousness but they are outside the reach of active control of the subject and thus are not located within the subject. They are given to the subject as constraints, and constitute an objective reality outside the subject.
Essentially, the fact that there is any given (obstructions, blockers to your free choices) to your experience points to something outside the subject.
You could argue that those obstacles/constraints/laws are still generated by the mind of the subject. Maybe I am the only mind that exists, and for some random reason, I am creating an experience which has that gravity constraint but there is no necessity to that. Or there are several isolated minds, who each conjure up their own "reality" where different rules may apply.
A reply to that could be a probabilistic argument: there are so many coincidences in what we perceive, and so much complexity, it seems unlikely a single mind could maintain that universe, and therefore there are probably things that exist independently of our minds but manifest themselves in our experience through all those coincidences and limitations to our will. Of course, we don't know that, but it would be a probable conclusion.