Why does somebody always have to say something like this in these threads? It’s out of place within the context of the question, which starts right out saying, “If God is real…” That says to me that the OP is asking us to assume, for the sake of argument, that the stated condition is true. Whether it is true or not is irrelevant to the question.
What if this was a math problem? “If a train leaves Triopolis at 50MPH…”, pointing out that Triopolis has no railroad service contributes nothing to solving the math problem. Ah, but we’re talking about religion here, so of course it’s imperative that we establish the mythological status of Bible stories. :rolleyes:
Anyway, to answer the OP:
Before Adam and Eve sinned, they were in direct, face-to-face communication with God. We can see this in the way the Bible describes God walking through the Garden to visit them after they had sinned - they heard Him coming, and they hid themselves. Had God simply been a voice coming from the sky, they most likely would have seen the pointlessness of hiding - they would have perceived Him as being too big to hide from. But since they were accustomed to visiting with Him face-to-face, of their human minds perceived Him as a person from whom they could hide.
The act (decision) of disobedience (eating the fruit) was indeed a rejection of God. At the very least, it was a rejection of His will. Because of that decision, God withdrew. He no longer walked and talked with them in the Garden on a regular basis. We could say that from that point on, God only showed up physically to make important announcements.
In any case, Adam & Eve knew who and what God was, and they knew what was expected of them. They knew the proper way to serve Him and worship Him. Even after He withdrew from them, they still knew what to do. This knowledge was passed on to Cain and Abel. However, Cain and Abel did not have the benefit of visiting with God face-to-face. They had to perform their service and worship based on what their parents taught them. And somewhere along the line, Cain misinterpreted those instructions. That was the source of his conflict with his brother: Able was following the instructions properly, and his work was acceptable. Cain, either intentionally or inadvertently, did not follow the instructions, and his work was unacceptable.
At this point, when Cain’s work was found unacceptable, he could easily have corrected his error (repented). He would have then been found acceptable. Instead of correcting his error, he believed he was being compared to his brother. He may have thought that, by eliminating his brother, there would no longer be a comparison, and his work would be acceptable.
And so error crept into the second generation. We can see, even today, that when an error is left uncorrected, it tends to multiply itself. It was the same then. Cain had received the proper instruction, but chose to do things his own way. It’s safe to assume that he then neglected to teach his own children the proper ways. In fact, he probably continued in his own error. His children then learned from him, and likely added their own errors into the mix. This continued, generation after generation.
Eventually, we had a large group of people who had no clue about the original instructions. And as more and more errors crept into their beliefs, new gods were invented, new religions sprang up.
At the same time, there were some who continued to adhere to the original instructions. And so we read the stories of those people. Although God had withdrawn physically from the human race, and no longer had a face-to-face relationship with the majority, there were still those who were willing to listen and obey. Noah was willing to listen, and was apparently the last of the original obedient line. So God spoke directly to him. “Build an ark!” But then the whole thing started over again - as the Earth was repopulated, errors once again crept into the beliefs, and a few generations later, there are new gods and new religions again.
So God came down and spoke to Abraham. I think the odds are that Abraham may have been an adherent of one of the false religions. But God recognized that Abraham would listen if spoken to directly. Abraham did, and so we read his story. In spite of the prevalence of pagan religions there were still those people here and there who continued to adhere to the original instructions. King Melchizadek, whom Abraham encountered. Moses met and married the daughter of Jethro, who was identified as a priest of God. There were always those for whom the original instructions had been passed down intact, even though everybody around them was screwing up.
An organized religion was not originally necessary. But eventually, it was needed. God decided that there needed to be a people established who would maintain the instructions. And so he arranged everything that let to the creation of the nation of Israel and what we now know as Judaism, and from Judaism sprang Christianity.
So God was “realized” right from the beginning, but it was the people who went astray.
Most religions, whatever god they worship, usually make it clear that if you were standing in the physical presence of their god, you would be left with no doubt as to who it was. But you’re correct: there is no test. It’s one of those situations where you’ll know it when you see it.