In John 7:46-52, we have evidence that Nicodemus' spiritual condition has changed since his first meeting with Jesus:
Finally the temple guards went back to the chief priests and the
Pharisees, who asked them, “Why didn’t you bring him in?”
46 “No one ever spoke the way this man does,” the guards replied.
47 “You mean he has deceived you also?” the Pharisees retorted.
48 “Have any of the rulers or of the Pharisees believed in him?
49 No! But this mob that knows nothing of the law—there is a curse on
50 Nicodemus, who had gone to Jesus earlier and who was one of their
own number, asked,
51 “Does our law condemn a man without first hearing him to find out
what he has been doing?”
52 They replied, “Are you from Galilee, too? Look into it, and you
will find that a prophet does not come out of Galilee.”
A man who would defend Jesus against the unjust anger of the leaders of his people is not a man without faith. Nicodemus did go to hear Jesus first to find out what he was doing. By his own standard, if he had found something wrong with Jesus' teachings and activities, he would have joined in with the accusers. He did not.
Later on, in John 19:38-42, Nicodemus helps Joseph of Arimithea prepare Jesus' body for burial. It says that Joseph of Arimithea asked for the body in secret because he was afraid, but lists him as a disciple. It does not specifically say there that Nicodemus was a disciple, but by including the two of them in the same account, it is reasonable to assume that Nicodemus was by that time also a disciple.
I am compelled to answer a narrower question: immediately following his meeting with Jesus, what evidence was there that he was NOT YET SAVED?
This question, I believe, has good answers.
There is no description of Nicodemus falling at Jesus' feet or immediately worshiping him in some other way. Compare to a number of people who were healed (the man possessed by Legion, the one leper in ten) or who had a family member healed (man with ill servant). They had an immediate reaction of gratitude and thanksgiving. We do not see that from Nicodemus.
Compare also to people who were forgiven: the woman at the well, the adulteress, the woman who washed Jesus' feet with her hair. They exhibited powerful emotions and displays of affection or allegiance.
He did not openly associate with Jesus as a public disciple (again, see John 19).
He did not ascribe divinity to Jesus as did Peter and others.
He did not immediately confess his sinfulness, as did Zacchaeus.
People had many different responses to Jesus to show the condition of their heart, but Nicodemus did not exhibit any of them at the first meeting except that he did not condemn Jesus, question his authority, or criticize his disciples. So he did not do what the wicked did or the righteous, which is most peculiar.
I read this article: http://sbctoday.com/13132/
This made me consider the story of the snake that Moses lifted up to save the lives of the Israelites bitten by snakes. Until and unless they looked at the snake in faith and confessed their sins, they were sick and would soon die. Since we have the story of Nicodemus helping to collect Jesus' body and prepare it for burial, I believe that the story of Nicodemus was intentionally left unfinished. He was told that the Son of man must be lifted up. Thus once Nicodemus saw Christ lifted up on the cross and intentionally went to see him in that state, he was saved, not before. Thus at the time when Nicodemus first visited Jesus, he was not saved, but when he saw Christ lifted up, he was.