One basically has to decide whether to trust, because one trusts the Church, that the tradition of venerating the saints going back to the age of the Ecumenical Councils is in continuity with the practice of the "primitive Church". Or whether to believe, based on an absence of written documentation in Scripture, "that Mary's sainthood and veneration were neither part of the primitive Church's phronema, nor internal or external teaching."
The issue of venerating saints was intensely tied up with the iconoclastic movement. There were three strains of iconoclasm, and one of the strongest strains was one made up of those whose objections to icons were intimately tied up with the belief that the Church had gone overboard with the veneration of the Theotokos and the saints. The declarations of the 7th Ecumenical Council and the writings of key figures surrounding the controversy such as St. Theodore the Studite and St. John of Damascus are very explicit affirmations of the beliefs and practices of the Church regarding the veneration of the saints.
That would be easy save for the fact that the Church is full of sinners rather than saints, Agrarian, and is broken up and disunited.