In your case it's you doing the checking. In every other case it's each individual doing the checking.
And why IT is the final authority.
Sorry, but the checker, each person in your system, is the final authority. That's why you have such offshoots going down such different roads using sola scriptura as their guide with each person authority on where the guide points. It just doesn't work, not for One faith, One Church anyway.
It's just woefully unworkable and impractical; which is why, IMHO, Jesus didn't establish His Church with this framework.
Roman Catholics will tell us that we need to consult the Magisterium in order to know what Scripture is, to understand it and to settle the various debates over its meaning and interpretation. But when we ask them why we should believe the Magisterium has the authority to establish the canon and produce the correct interpretations of Scripture, we are often treated to a series of Scriptural proofs, which presuppose the Scriptures are clear and authoritative. Whitaker observed this in his own day, and noted how this kind of argumentation is viciously circular (emphasis mine):
For I demand, whence it is that we learn that the church cannot err in consigning the canon of scripture? They answer, that it is governed by the Holy Spirit (for so the council of Trent assumes of itself), and therefore cannot err in its judgments and decrees. I confess indeed that, if it be always governed by the Holy Spirit so as that, in every question, the Spirit affords it the light of truth, it cannot err. But whence do we know that it is always so governed? They answer that Christ hath promised this. Be it so. But where, I pray, hath he promised it? Readily, and without delay, they produce many sentences of scripture which they are always wont to have in their mouths, such as these: "I will be with you always, even to the end of the world." Matth. xxviii. 20. "Where two or three are gathered together in my name, there I will be in the midst of you." Matth. xviii. 20." I will send to you the Comforter from the Father." John xv. 26. "Who, when he is come, will lead you into all truth." Johnxvi. 13. I recognise here the most lucid and certain testimonies of scripture. But now from hence it follows not that the authority of scripture depends upon the church; but, contrariwise, that the authority of the church depends on scripture. Surely it is a notable circle in which this argument revolves! They say that they give authority to the scripture and canonical books in respect of us; and yet they confess that all their authority is derived from scripture. For if they rely upon the testimonies and sentences of these books, when they require us to believe in them; then it is plain that these books, which lend them credit, had greater authority in themselves, and were of themselves authentic.1
Some Catholics, such as John Salza, have attempted to avoid this vicious circle by countering that such an appeal to Scripture is spiral, not circular:
When Catholics explain that we believe in the Bible on the authority of the Catholic Church, Protestants accuse us of circular reasoning. They say we get this information from the Bible and so the Bible, not the Church, is the final authority. This argument, while clever, is incorrect. The Catholic argument is what we would call spiral, not circular. First, the Catholic approaches the Scriptures as historical books only, but not inspired. Based on the historical evidence, the Catholic establishes the Scriptures are authentic and accurate documents. Second, the historically accurate Scriptures reveal that Jesus established an infallible Church based on texts like Matthew 16:18 and 1 Timothy 3:15. Third, this infallible Church has determined which Scriptures are inspired and which ones are not. Based on the authority of the infallible Church, the Catholic believes in the inspired Scriptures. This is the only logical and rational approach to accepting the inspiration of the Scriptures, and this is John Salza with Relevant Answers.2
As I understand it, Salza wants to move from demonstrating the Scriptures as historically accurate to demonstrating that these Scriptures attest to an infallible Magisterium. We then turn to this Magisterium to know that the Scriptures are inspired:
historically accurate Scriptures --> infallible
Magisterium --> inspired Scriptures
Salza's reply is interesting, but there are a number of problems:
i) There's nothing intrinsic to historical cases for the historical accuracy of Scripture that limits such an appeal to Catholics only; Protestants are free to make the same historical case as well.
ii) Apropos, the move from historical accuracy to inspiration is exceptionally short. The difficult components of any external demonstration of inspiration are in establishing the historical accuracy of the New Testament documents. But once that is accomplished, it is a much simpler matter to move from the historical fact of the Resurrection, which establishes Jesus as God, to the ministry of the Holy Spirit, which gives inspiration to the Scriptures. If the Magisterium isn't needed to demonstrate the much harder case of historical accuracy, it's hardly required to demonstrate the much easier case of inspiration.
iii) I don't even know how, in principle, you can divorce historical accuracy from inspiration. A good deal of the data contained in Scripture cannot be both accurate and uninspired, e.g. various prophecies, knowledge impossible to discern in any natural method (what someone or some group was thinking in their hearts at one time or another), what God was doing, thinking or intending, etc. And some data, even if they are knowable through natural methods, carry a certain theological significance that could not be accurately known (as truth) by the authors of Scripture without inspiration.
This is also why there is generally a correlation between denying historical accuracy and denying inspiration. The two go hand-in-hand.
iv) How can Salza establish the Scriptures as authentic and accurate documents if we need the Magisterium to interpret those very documents for us? If the Scriptures are unclear or difficult to understand, as Catholics often assert, this would apply whether or not they were inspired.
v) If we can properly interpret all of the passages required to make a case for the historicity of Scripture (e.g. the Resurrection being supported by 1 Corinthians 15) before we establish the Magisterium as authoritative, why do we need the Magisterium to properly interpret all of Scripture once we learn that it is inspired? If we were competent enough to interpret the Scriptures before we discovered their historical accuracy, we should be competent enough to interpret them afterward.
vi) His appeal to Matthew 16:18 and 1 Timothy 3:15 is dubious (see here for a short, but devastating critique of appealing to 1 Timothy 3:15; the comments section also contains links to discussions of Matthew 16:18).3 So even if the circularity is avoided by this argument, the Scriptures still do not establish an infallible, authoritative Catholic Magisterium.
http://beggarsallreformation.blogspot.com/2010/07/vicious-circle.html