If we are only ever subjective creatures, how do we know there is objective reality?

We don’t. This is the point I already yielded to postmodernism. We cannot know for sure. We have to take the existence of an objective reality as an act of faith. But it’s important to be clear on this: presuming objective reality does not exist is also an act of faith. They are both acts of faith.

How can we measure how objective we are truly capable of being?

We can’t. But we can definitely look at the efficacy of the presumption of an objective reality. People who presumed an objective reality invented your phone.