How did people know that their visions of God were not false before the Bible was as a book compiled? I looked at many Christian apologist websites, like GotQuestions, which states that anybody who claims to see God can have their claims evaluated by Scripture. So far so good, but how would people like Abraham know that their visions of God were real and that they were not deceived if they didn't have the complete edition of the Scripture?
The first means is fulfilled prophecy. 


Methusaleh's name was a prophecy: it means "When he dies, it will come". The year he died, the flood of Noah arrived. 
God told Abraham that he would have a son. Twenty-five years later, he did.
Elihu announced the coming of God to Job, and God came.
Job uttered at least two dozen prophecies, which ultimately came true.
Joseph had dreams, interpreted them correctly, and the events came to pass.
Moses asked God for a sign that he would succeed in rescuing his people from Egypt:




  12 And God said, “I will be with you. And this will be the sign to you
  that it is I who have sent you: When you have brought the people out
  of Egypt, you will worship God on this mountain.” (Exodus 3:12)



The second means is miracles.


Enoch did not die, but was taken up into heaven
The wombs of all the women in Pharaoh's household were first shut, then opened because of how he treated Abraham
The plagues against Egypt certified what God told Moses
Job's healing and the restoration of his fortune and long life certified what God told Job

The third means is righteous character and loyalty to God in the midst of suffering and persecution:


Joseph refused to sleep with his master's wife while a slave.
Job was already a righteous man, yet when he suffered, he did not curse God.
Lot maintained his character and hospitality though surrounded by a completely wicked culture.
Abraham refused to accept the spoils of war from the unrighteous kings he was forced to ally himself with.

A consistent message, regardless of the medium of communication emplyed by God. In my study of Job, I counted thirteen different means of communication employed by Job and God to communicate with one another, and the different pieces assembled from all those means fit together harmoniously. 
Faith in God despite impossible circumstances.


Abraham waiting all those years for a son, then being asked to sacrifice him
Jacob, enduring years of false dealing by his uncle Laban
Joseph in prison, believing God would set him free
Job, on the verge of death, believing that God will raise him from the dead
Moses before Pharaoh, believing God would set his people free

The Holy Spirit. The Spirit grants wisdom and many other gifts. When a life displays that fruit, it is persuasive.