How to determine a genuine call from God? Imagine one day a person hears voices like God speaking to him (or as how he would describe as God), with bright lights, angels, burning bushes, and all. It would seem so real for him but only he alone heard or experienced it. And the voice would tell him to sacrifice his son/daughter to God.

Is it moral for him sacrifice his child because of this experience? it might be a test and if he didn't then he would have failed for doubting God. What would a Christian do?
We have Scripture by which we can test these revelations.


  “The prophet who has a dream, let him tell a dream;
  And he who has My word, let him speak My word faithfully.
  What is the chaff to the wheat?” says the Lord.
  “Is not My word like a fire?” says the Lord,
  “And like a hammer that breaks the rock in pieces?
  (Jeremiah 23:28-29)  


God calls people within the context of His revelation. If a person tells me out of the blue that God told him this and that, I have no reason to believe him; but if this call came from a context of God showing His presence and power, I need to look more closely.

Since the question refers to Abraham's willingness to sacrifice his son, lets look at the context of that command:


God guided him from his homeland to the place promised to his descendants (Genesis 12)
God promised him a son, though Abraham and Sarah were too old (Genesis 17)
The Lord appeared to Abraham, and Sarah also saw and heard Him (Genesis 18)
The destruction of Sodom and preservation of Abraham's nephew Lot (Genesis 19)
God preserves Sarah from being violated as a result of Abraham's lie (Genesis 20)
A nonbeliever converses with God in a dream concerning Abraham and Sarah (Genesis 20)
Sarah has a son! (Genesis 21)


All of this comes before God tells him to sacrifice his son. God has proven himself to Abraham; Abraham obeyed because he trusted God.

It's much the same way with Jesus -- if you walk up to me and tell me that you are the son of God, I'm not going to believe you -- without anything in your life to support the claim, how can I believe it? But the context of Jesus' life makes it possible (even probable). God reveals Himself, then wants us to trust Him. He doesn't normally demand obedience without showing Himself to be trustworthy (though He does have every right to do so).