1) The first thing to be used for the discerning of saints is heresies. Heresies are the friends of the saints, they are to help us know who are of us and who are not. Were there not abundant heresies out there it would be even more difficult determining who are brethren and who are not (1 Cor 11:19).
2) The next thing to be used for the discerning of saints is “What think ye of Christ?” Not so much in regards to heresies, but rather how does one think of Him personally. Some refer to Him as Master, which in every incident in scripture is a sign of unbelief in whole or in part. Those in this group often refer to themselves as slaves, and know not that “thy maker is thine husband.” Others play the harlot with their husband and have many lovers among the world, the flesh and the devil. Not so of the saint, who though they may fall through weakness of the flesh, yet they hate it and their love however feeble, is still for Christ alone. Christ is the Husband of His bride, and His bride will receive Him and love Him as such.
3) Another thing to be used for the discerning of saints is a humbling. Has the person been humbled? Note: It is not “are they humble?”, but rather “have they been humbled?” When one has been met by God, it leaves them humbled. When one has been humbled they talk not about their practical holiness, nor their value before God, they simply take the lowest place among men and leave any inviting to a higher place to Christ (Luke 14:8-11). Similar to Isaiah who declared “Woe is me! For I am undone; because I am a man of unclean lips, and I dwell in the midst of a people of unclean lips.” The reason he said this follows, “for mine eyes have seen the King, the Lord of hosts.” Or as Job states, “Behold, I am vile; what shall I answer thee? I will lay mine hand upon my mouth.” A sight of the King leaves no room for exaltation of self, and one willingly takes their place among the lowest of men. Those who claim to be humble, have never been humbled, for a claim of humility is not needed in the humbled.
4) Another thing to look for is If one has any comfort in any sin, whether lying, adultery, fornication, formality, morality, hypocrisy. Note: The saved can commit nearly any sin that a lost person can, yet they cannot have peace in it, nor can it be a defining characteristic of themselves. It is possible for one to have the comfort caused by ignorance of the sin as David did, but once the sin is exposed, like David, the saint can have no comfort in it.
5) Being unable to discern a child of God is a quality of the lost, who being lost, have no discernment (Prov 17:15, Jonah 4:11)
6) Next we are to keep an eye out for the obvious bruised reeds and the smoking flax, for which one needs the utmost wisdom and guidance of the Spirit of God. “And of some have compassion, making a difference: And others save with fear, pulling them out of the fire; hating even the garment spotted by the flesh.” To exercise this we reserve compassion for the bruised reed, and fear of judgment for the smoking flax, and see with which response it is met. Look not only on outward actions or professions to determine the brotherhood of any, but especially these, for their outward signs may well be contrary to their inward birth.
7) Those led of the Spirit of God will be found to be walking by faith, not by sight. They may be called to do things against both caution and reason, and will generally be found swimming upstream in the current of their surroundings. They will be considered foolish (1 Cor 2:26-29) yet in their workings will be found the wisdom of God (1 Cor 2:11-14).
8 ) People who are led of a certain spirit will recognize and prefer others led by that same spirit. Those led of the spirit of the world will prefer others led of the spirit of the world; those led of the spirit of whoredoms will prefer others led of the spirit of whoredoms; those led of the spirit of religion will prefer others led of the spirit of religion; and those led of the Spirit of God will prefer others led of the Spirit of God. Fellowship with others led of the same spirit as yourself will not be awkward, but comfortable, even preferable. In this way, those who are led of the Spirit of God will be able to discern others who are led of that same Spirit (1 John 4:5-6).
There are others as well as I am sure you will find, the point of the matter is that the child of God is meant to know who his brethren are, and is meant to judge rightly.
Prov 17:15 He that justifieth the wicked, and he that condemneth the just, even they both are abomination to the Lord.