It doesn't say that spiritual discernment is necessary to identify a Christian, no. Not directly. But if the Bible is the guide for what fruits are good, and spiritual discernment is necessary to interpret the Bible, then spiritual discernment is necessary for identifying good fruits. Therefore if good fruits are necessary for identifying Christians, and spiritual discernment is necessary for identifying good fruits, then spiritual discernment is necessary for identifying Christians.
The Bible says what Christians should do. If I can't interpret the Bible, then I can't tell whether a Christian is doing what a Christian should do.
Try this. Tell me how to know what is a good fruit, without telling me to consult the Bible that I can't interpret.
I think we need to deliberate the above in the various perspective, i.e.
Who is a Christian - In God's view
Who is a Christian - In Christian or non-Christians view.
Who is a good Christian - In God's view
Who is a good Christian - In Christian or non-Christians view
]
Who is a Christian in God's view is a person who had accepted God's offer re John 3:16 to enter into a covenant [implied or explicit] to comply with the covenanted terms.
This is not an issue in principle, because God being all-knowing would know the sincerely and truth of the Christian who accepted God's offer.
Who is a Christian - In Christian or non-Christians view.
In this case the person may personally declared his acceptance of God's offer an in John 3:16 and went through the baptism ritual and perform all the obligation that can be verified empirically.
But the fact is, despite all the above external empirical verifications and good fruits, no human [not all knowing and limited] can confirm whether the person who declared himself is truly a genuine Christian or not. Only the all-knowing God will know.
Who is a good Christian - In God's view.
God being omniscient or all-knowing will certainly know who is a good [in a range of degrees] Christian.
Who is a good Christian - In Christian or non-Christians' view
As with the above,
all the above external empirical verifications and good fruits, no human [not all knowing and limited] can confirm whether the person who declared himself is truly a genuine Christian or not. Only the all-knowing God will know.
Whilst Christians and non-Christian can discern and verify the acts of a Christians against what the NT describes as good works, what is necessary is, whatever the conclusion of a good Christian, it must not be declared with 100% certainty merely based on interpretations from verses in the NT.
The point is there are many
mental conditions and states within the person's mind or "heart" [which only God knows] that humans [Christians and non-Christians] cannot discern, e.g. real agape, grace, faith, reverence for God.
There are many cases of Christians who had expressed and performed acts that is very Christian-liked. However at some point, they were proven with evidence to be pedophiles priests, homosexuals, serial killer [BTK], criminals and evil people. Example BTK - Dennis Rader the serial killers;
Dennis Lynn Rader (born March 9, 1945) is an American
serial killer known as
BTK or the
BTK Strangler. He gave himself the name "BTK" (for "Bind, Torture, Kill"). Between 1974 and 1991, Rader killed ten people in the
Wichita, Kansas metro area. -wiki
Rader was a member of Christ Lutheran Church and had been
elected president of the church council.
-wiki]
I am sure many of the church members in his congregation would have the idea that Rader was a very good Christian and elected him to be their president based on the good works and fruits he had produced based on empirical evidence of work done by him for the church and members.
To a certain extent, Christians can discern who is a 'good' Christian to the best of their ability based on what is stated in the NT, but they should never be 100% or even 90% certain since they are fallible human beings. Only God being all knowing can be 100% certain.
Thus whatever the conclusion of who is a "good" Christian arrived by Christians or non-Christians who are fallible human beings can only be subjective.
One point is;
If a person who had entered into a covenant with God commit a killing or violence again non-Christians or other Christians, it is conclusively certain within the framework of Christianity and the covenant, that act has nothing to do with Christianity in relation to those violent acts of killing.
Since the person has entered into a genuine covenant with God, he is still a Christian by definition and it is up to God to judge to forgive him or not on Judgment Day.