Do you have an objective test, and what it is? I don't think people have any objective test. To me closest to that is, if other people can see the same, it is probably objectively true. But event that can be said to be not purely objective.
For me, truth is a quality of statements, and a statement is true to the extent that it accurately reflects / corresponds with objective reality. (That's often called the "correspondence" definition.)
As you say, subjectivity can often be a problem for humans. That's why traffic cops use cameras, for instance. It's also why, if someone claims that unicorns, or Bigfoot, (&c &c) are real, the appropriate response will usually be, "Show me."
It's also a problem God has, since [he] never appears, never says, never does, and is only known to exist as a concept in an individual brain (overall, a wide variety of concepts in individual brains).