There are some Scripture versions that attempt to put Yahweh's Name back into the text in every instance where they believe it had been replaced. My question is, how can they know for sure whether the original actually used His Name "Yahweh" or if it simply said "God"?
ANSWER:
The Hebrew word generally translated "God" is "Elohim", or "Mighty One(s)". It is true that we cannot correct every instance of "God" with Yahweh, as most instances of "God" would more correctly be "Elohim". Most times, in the TaNaK ("Old Testament"), wherever "the LORD" (in all caps) is printed, it was originally YAHWEH. There are also instances where GOD is in all caps, denoting the original usage of the Name Yahweh. In KJV, "Yahweh" was replaced 6043 times with "the LORD", 365 times with "LORD", 309 times with "GOD", 107 times with "the LORD's", 4 times with "Jehovah", and 1 time with "LORD's". (Refer to the Strongest Strong's Exhaustive concordance, Hebrew word # 3068.) Additionally, the shortened form of Yahweh (the Hebrew letters "yod" and "heh", spelling "YaH") was replaced 45 times with "the LORD", and 3 times with "LORD". (Refer to the Strongest Strong's Exhaustive concordance, Hebrew word # 3050) In one instance, it was actually written "Jah". Psalm 68:4, KJV ~ "Sing unto God (more correctly, "Elohim" / "the Mighty One"), sing praises to His Name; extol Him that rideth upon the heavens, by His Name Jah (written, Iah in the 1611 KJV), and rejoice before Him."
Where God is not in all caps, it most likely refers to the Hebrew word "Elohim" (Mighty One). Where Lord is not in all caps, it most likely refers to the Hebrew word "Adonai" (Master).
Quite a few Bible prefaces will tell you the basic guidelines for how they translated Yahweh, Elohim, Adonai, etc. Also, if you look it up in the Strong's Hebrew, or in an interlinear, it will tell you which instances were "Yahweh", "elohim", "adonai", etc. You may also be interested in ordering a copy of "The Scriptures", available from Fossilized Customs This uses Yahweh's Name. However, "the Scriptures" version does not use the Peshitta for the Apostolic Writings, so it is mainly more helpful for the TaNaK.
As far as the Apostolic Writings go, many times when they read "the Lord", it is in fact supposed to be "Yahweh". The Aramaic Peshitta, which was preserved by the Church Of The East, has preserved the correct rendering of where it's supposed to be "Elohim", "Master", or "Yahweh" . Yahshua's Name is preserved in this text too. You may be interested in a copy of "The Word Of Yahweh", available from Assembly of Yahweh - 1017 N. Gunnell Road - Eaton Rapids, MI 48827 USA, and also "Hebraic Roots Version Scriptures", available from Fossilized Customs. Both of these versions use the Peshitta to determine the use Yahweh's Name in the Apostolic writings.
You may wish to check out www.aramaicpeshitta.com for further explanation on the Peshitta. There is an interlinear on this site, and regrettably, it replaces Yahweh's Name with "the LORD". This site may have some information on the Aramaic Peshitta, though, which you would find helpful. And the three Scripture versions mentioned above would probably be a great help in determining where the Name is to be placed, as they do use Yahweh's Name.
For more information and excerpts from Bible prefaces, etc., you'd probably be blessed by checking out Why The Names Yahweh and Yahshua?