Who determines what is sound and what is shoddy?
Not the JW Governing Body, RC Pope, LDS Prophets, etc.
It can be objectively shown that Watchtower scholarship is shoddy. They misquote trinitarians to try to disprove the trinity (?!), add or subtract words from the Bible, use specious, obscure sources to try to prove their point, while ignoring a wealth of defensible, credible scholarship. They make glaring mistakes even inventing non-existent Greek tenses to try to support their view (e.g. Jn. 8:58).
They flip flop, change like the wind, make false prophecies, have used pyramid measurements to set end time dates (?!), quote spirit mediums to support doctrine (J. Greber).
How do you know that the Trinity, Deity/virgin birth/incarnation/resurrection, etc. of Christ is true? We can know these things are true without being KJV-only. JWs and Mormons quote the KJV, yet are heterodox on salvific points.
We can show that Jn. 1:1 is God, not 'a god' based on grammar, context, theology, etc. JWs are right that the correct interpretation of this verse means our salvation (1950 booklet they publish), but they are wrong about their translation and denial of His Deity. Good research establishes this, while their arguments for their view can be dismantled. Truth can be proven and error can be refuted. The authority is in the Word of God, but it must be properly translated and interpreted.