"How do I know that the Council of Chalcedon is a valid Council from an Orthodox perspective for example? How could I recognize Truth in the battles of Arianism (even Constantinople suffered from this?)."
Ecumenical means that the entire Church throughout the world accepts it. That makes it valid. OOs would disagree, but whatever.
Christ said he'd lead His Church into all truth.
St. Irenaeus said: "The Church, having received this preaching and this faith, although scattered throughout the whole world, yet, as if occupying but one house, carefully preserves it. She also believes these points [of doctrine] just as if she had but one soul, and one and the same heart, and she proclaims them, and teaches them, and hands them down, with perfect harmony, as if she possessed only one mouth. For, although the languages of the world are dissimilar, yet the import of the tradition is one and the same. For the Churches which have been planted in Germany do not believe or hand down anything different, nor do those in Spain, nor those in Gaul, nor those in the East, nor those in Egypt, nor those in Libya, nor those which have been established in the central regions of the world... For the faith being ever one and the same, neither does one who is able at great length to discourse regarding it, make any addition to it, nor does one, who can say but little diminish it." (Against Heresies I, 10, 2)
It's through the concrete unchanging bulwark of Holy Tradition, and through the consistent testimony of Apostolic Succession and the oversight of the laity of the Church, isn't added to, or taken away from.
Tradition is static in Orthodoxy, it doesn't 'evolve.' The councils simply affirm what the Church's stance has always been.
Chalcedon is one of history's most confusing events. Essentially, the OOs held to the Fathers (St. Cyril of Alexandria), and the EOs held to the Councils (Chalcedon). I think that's a good way of putting it, but that may generate some controversy. I don't think there is a good way to talk about Chalcedon without having polemical discussions.