Can the belief of Jesus' listeners be considered belief unto salvation? During my daily study of the book of John, I've come across several statements like this one from chapter 8:30, which says:


  "As he was saying these things, many believed in him."


Is there any biblical, or even logical, argument to conclude that this belief was unto salvation, or anything that could be used to cast credible doubt on this?
Can the belief of Jesus' listeners be considered belief unto salvation? 

YES


  Galatians 3:2-7 (NLT)
  
  Let me ask you this one question: Did you receive the Holy Spirit by
  obeying the law of Moses? Of course not! You received the Spirit
  because you believed the message you heard about Christ. How foolish
  can you be? After starting your Christian lives in the Spirit, why are
  you now trying to become perfect by your own human effort? Have you
  experienced so much for nothing? Surely it was not in vain, was it? I
  ask you again, does God give you the Holy Spirit and work miracles
  among you because you obey the law? Of course not! It is because you
  believe the message you heard about Christ. In the same way,
  “Abraham believed God, and God counted him as righteous because of
  his faith.” The real children of Abraham, then, are those who put
  their faith in God.


Abraham was counted as righteous because of his faith. In the same manner, those who believed in the words of Jesus are cleansed through the Word of God.


  You are already clean because of the word I have spoken to you. (John
  15:3, NIV)


In passage you posted(John 8:30), Jesus was telling to the people that He is the Light of the world and the promised savior, who will save us from our sins. All they had to do was believe in Him and the Father who sent Him, and they will not die in their sins. 


  John 8: 24-30 (ESV)
  
  I told you that you would die in your sins, for unless you believe
  that I am he you will die in your sins.” So they said to him, “Who
  are you?” Jesus said to them, “Just what I have been telling you from
  the beginning. I have much to say about you and much to judge, but he
  who sent me is true, and I declare to the world what I have heard from
  him.” They did not understand that he had been speaking to them about
  the Father. So Jesus said to them, “When you have lifted up the Son of
  Man, then you will know that I am he, and that I do nothing on my own
  authority, but speak just as the Father taught me. And he who sent me
  is with me. He has not left me alone, for I always do the things that
  are pleasing to him.” As he was saying these things, many believed
  in him.


Though Jesus was not yet crucified, faith in Him was sufficient to have salvation. For,  The death of Jesus saves all the believers before and after His crucifixion.