We're not talking here about peering into men's consciences. You can't do that, and it's not required anyhow. When you come to apostolicity, use the historically unbroken descent of the Catholic Church and use Rome as the central peg. If you have described the marks well, there won't be any question about their existence. Then you have to show what their existence proves.
Miraculous Unity Look again at unity and catholicity, which can be considered together.
The key here is miracles, because these marks are miracles. They can't be accounted for any other way. The Church has been unified throughout the centuries, teaching one doctrine. True, individuals Christians have lost that unity, going this way and that, sometimes doing so corporately in the form of sects that split off from the Church. But the Church itself has always remained one, no matter how many have left its unity.