You can know that Mark founded the church in Alexandria the same way you can know that Homer wrote the Iliad or Cyrus was the king of Persia: by using accepted historiographical rules of evidence: letters, chronicles, inscriptions, monuments,lists of people commemorated in public celebrations, proclamations of a legal nature, etc.
The writings of people like Eusebius, the inscriptions on the walls of the catacombs, the names listed in the anaphora --- these would be good examples.
BTW, the whole idea that Mark WROTE the "Gospel according to St Mark" comes from this same capital-T Tradition. You won't find the authorship of any of the for Gospels attested to in the Bible itself.
Permit me to excuse myself now from this argument. I need to go back to sleep.
Please pray for me, a sinner.
I just read you are ill and will of course pray for you.
There is no one on FR that I admire more or learn from than you. So you must get better to continue to post here to help me grow in my Christian walk!! ( no pressure )