Summary
As may be seen in the tags, "trinity" and "eisegesis" the OP believes any reading of the Hebrew Bible understood as agreeing with the Christian belief of "Trinity" is reading an unfounded belief into the text. Hence this question:
How do trinitarians know that the verses of the Hebrew Bible which make positive claims about the "oneness of God" are in fact only referring to the "being of God"?
The Christian canon is both Old and New Testament and the Old is read in light of the New. As Paul states, the Old has divine actions which are hidden until they are fulfilled. For example, when Genesis reports God as saying, "Let us make man in our image and after our likeness..." the Christian looks beyond an eisegetical divine council and understands the plural pronouns include both God the Father and the Son of God.
1 Corinthians 8:6
yet for us there is one God, the Father, of whom are all things, and we for Him; and one Lord Jesus Christ, through whom are all things, and through whom we live.
The plural "us" is explicit, yet the identity of "us" is hidden until the Son reveals the Father. The inability to differentiate or speculate which person or which being cannot erase the explicit oneness in God's action in creating man.
Arguably, the Christian explanation of "person" follows passages in the Hebrew Bible which describe "personal" encounters with God. Labeling them as a "theophany" is a convenient way to side step the very issue at the heart of this question. The Living God's personal involvement with creation, and specifically taking action to redeem man from sin should not cause one to see those actions as conflicting with oneness, or being, or nature.
When Moses calls God a "Rock" we do not take this out of context to consider how the being of God as "Rock" is conflicting with God's oneness. To the contrary, a Trinitarian looks into the hiddenness of the Hebrew text in order to see what Moses saw.
1 Corinthians 10:4
and all drank the same spiritual drink. For they drank of that spiritual Rock that followed them, and that Rock was Christ.
Seeing what is living in words which narrowly describe an inanimate rock is not eisegesis.
The Triunity of God
The Old Testament has ample evidence describing the oneness and being of God.
- The word most frequently understood as God is אֱלֹהִים, Elohim which is plural. Inherent to Elohim is the ability to understand that which is plural is at the same time one.
- There are three words understood as God. In addition to Elohim, there is אֱלוֹהַּ, Eloha the singular from which the plural Elohim is derived and אֵל, El. The understood oneness of the plural Elohim does not preclude one acting as one.
- There are three "personal" names for God, יֵהוּא, יֵה, and היה. Moses who saw God face to face knows Elohim as היה and יֵהוּא and יֵה.
Overall, there is a triad of types of identification, two which have a triad of different terms.
What is present in the Old Testament as a whole is first made explicit when Elohim self-identifies in Genesis using first-person plural pronouns three times.
Genesis 1:26-27
Let us make man in our image and after our likeness...God created man in His own image; in the image of God He created him; male and female He created them.
An initial reading raises the essence of the OP's question: how does oneness not conflict with us and our? Obviously, one could understand "Us" as God and the Spirit of God (cf. Genesis 1:2) and have a satisfactory explanation. But this conflicts with the being and oneness of "God" demanded by those who reject the New Testament. Therefore eisegesis is necessary: one God consulted with a divine council before making man.
Trinitarians do not ignore the New Testament in order to narrowly understand the Hebrew Bible. To the contrary, they understand the New as revealing that which was hidden in the Old Testament. Thus, from a New Testament perspective, the pronouns include the "oneness" of Father and Son who, despite being "singled out" are "one" (cf. John 10:30).
"Let us, Father and Son speaking as one (cf. John 16:13), make man in our image and after our likeness..."
Ironically, this branded as eisegesis because the plurality must be a divine council. Why? Because when reading the Hebrew Bible one cannot distinguish positive claims about the oneness of God when it is possible to to see a text as referring to a divine council not the being of God.
Significantly the plurality of verse 26 is continued in the actions in verse 27.
- The plural in the verb asa (1:26) is replaced by bara which is singular but unnecessarily repeated to describe creation as three actions.
- The subjects of bara are God, He, He, following the pattern of the pronouns us, our, our. This too is an unnecessary element of the composition, unless the plural declared continued to act as one in the creating.
A careful reading of Genesis 1:26-27 supports understanding what is plural acting as one. Since the plans and the description of creating man are given in terms of three the implication of plural is three but at the same time described and able to be understood as one.
- Three first-person plural pronouns
- Three uses of the verb bara (not the asa)
- Three subjects for bara
"Let us make...our image...our likeness...God created...He created...He created" highlight an inherent misconception in the question. The text demands both plural and singular. Genesis 1:26-27 is the defining description of unity or oneness.
Applying Triunity
When later passages speak of "oneness" they are reminders of the definition from Genesis. Any passage which lacks corroborating action is a reminder, not a new definition.
Deuteronomy 6:4
Hear, O Israel. The LORD our God, the LORD is one.
On the other hand, when action is present, the plural is present.
Deuteronomy 32:39
"See now that I, even I, am he, and there is no God beside me; I kill and I make alive; I wound and I heal; and there is none that can deliver out of my hand."
This passage employs the same pattern as Genesis. It begins by God self-identifying using two singular first-person pronouns, אני אני הוּא.The unnecessary repetition, I, I AM describes that which is plural acting as one. It parallels Let us...
The initial declaration of self is followed by two first-person plural actions: I kill and I make alive...I wound and I heal. These parallel our image...our likeness. Here, as in Genesis, we see Elohim self-identifying as plural and describing an individual's actions in plural terms.
Conclusion
The definition of unity is given in Genesis 1:26-27: understanding the tripartite God means to see that which is plural acting as one.
Old Testament passages like Deuteronomy 32:39 describe God as plural, I, I AM... coupled with two dual actions, kill and make alive and wound and heal. Question which is which or "being" or "person" cannot erase what is clearly described as plural acting in oneness.