I did much research myself on this area.. I am not KJV only either because it is just a translation of the Scriptures and I had found myself some errors in KJV.
What really matters is not the translation, but the source manuscript used for translation. I am not concentrating more on OT because we have Septuagint translated into Greek 3 centuries before Christ... so technically, no one can change any words and we have two sources to verify which is older that Christianity itself.
However, NT has a very different story. Manuscripts are widely divided into 5.
Wiki:
Categories_of_New_Testament_manuscripts
- Alexandrian
- Egyptian
- Eclectic
- Western
- Byzantine
KJV is not translated from any of these manuscripts but Textus Receptus (or received text) what has been used and found in churches of that time and closely related to Byzantine manuscripts.
However, NIV and a host of modern translations are from Alexandrian manuscripts and are much older than Textus Receptus.
The real problem is finding out which manuscript is the actual fact and truth. One way that I used to find is another 2 supporting verses.
By the mouth of two or three witnesses every word may be established:
Matt 18:16 "But if he will not hear, take with you one or two more, that 'by the mouth of two or three witnesses every word may be established.'
2Cor 13:1 This [will be] the third [time] I am coming to you. "By the mouth of two or three witnesses every word shall be established."
1Tim 5:19 Do not receive an accusation against an elder except from two or three witnesses.
Scripture itself helps us how to find the truth: By the mouth of two or three witnesses every word may be established
(this again must must be established by three witness which I had showed above)
So, any contradicting verse in any translation must be supported by atleast two verses.
Comma Johanneum (1 John 5:7) is a well known example. In order to prove whether 1 John 5:7 really exists or not, we need to take the whole context, understand them properly and then find two other verses that prove the verse.
So, what do we need to prove if 1 John 5:7 is true. [For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one], We must prove Father, Word (or the Son). Holy Spirit bear witness - for who/what? Jesus Christ (refer verse 6).
- Father bears witness of Son: John 8:18
- Son bears witness of Himself: John 8:14,18
- Spirit bears witness of Son: 1John 5:6
Jesus Christ referring as 'Our witness' in John 3:11.
Next, we need to prove Father, Word, Holy Spirit are one.
- Father is the Son: Isa 9:6
- Son is the Holy Spirit: Acts 20:28
- Holy Spirit is the Father: John 4:24
So, it's done.. By three witness verses according to the Scriptures, I can say, '1 John 5:7' must exist in original scripture. Similarly we must do for all contracting verses in manuscripts.