In truth, there isn't a verse that explicitly states that only scripture is to be depended on.
From the beginning, Prophets and Prophetesses were God's representatives on earth, to declare the word of God. However, since the end of period of the Judges, and the installation of Kings, the main role of Prophets were to call Israel and others to repentance.
With the coming of the Apostles, they became God's representatives as you well know (and on that basis, the Apostoilic Succession, which I do not believe in).
But lets assume that there is a succession, or the Sensationalists are correct (that God continues to give us prophets and apostles), both the speaker, and the letter, cannot contradict (Galatians 1:9, Deuteronomy 18:19-23, & Isaiah 41:21-24).
And given the reality we live in (2 Corinthians 4:4, 11:13-15, & 1 John 4:1-2), the sure way to determine truth is through the word of God.
I'll be back later.