Basically just a gut feeling. Check out post #66. Even if MacArthur only refines and reinforces the converted, that is of value. But I know that people who reach the unsaved are more subtle. Someone posted recently about a successful method used by people evangelizing Muslims. It starts with what the Koran says about Jesus, and works from there. I.e. it meets people where they are. Jesus seldom went to people who were sincerely seeking and told them they were going to hell; he was more subtle, because God is not desirous that anyone be condemned.
I tend to avoid assessing the fruit [or lack there of] of a person’s ministry based on “gut feelings” - I am sure God is not honored when you belittle the ministry of one of God’s preachers based on your “gut”.