I take their confession by faith.
if someone confesses Jesus as Lord and Savior, I believe they are saved.
The reason for this is simple; I have faith, and am planted in the Lord Jesus. In addition to this, it is better for me to believe, and to pray, rather than to seek out evidence beyond the fruit of the Holy Spirit. If someone professes to believe in Jesus, yet has no fruit, in this case I would pray that they would bear fruit.
Now, some are indeed false brethren; the Lord knows who they are, and they will be exposed one way or another, and I pray for them also, that they would come to repentance, receive forgiveness, and be redeemed:
Jude 4
For there are certain men crept in unawares, who were before of old ordained to this condemnation, ungodly men, turning the grace of our God into lasciviousness, and denying the only Lord God, and our Lord Jesus Christ.
2 Timothy 3:1-13
3 This know also, that in the last days perilous times shall come.
2For men shall be lovers of their own selves, covetous, boasters, proud, blasphemers, disobedient to parents, unthankful, unholy,
3Without natural affection, trucebreakers, false accusers, incontinent, fierce, despisers of those that are good,
4Traitors, heady, highminded, lovers of pleasures more than lovers of God;
5Having a form of godliness, but denying the power thereof: from such turn away.
6For of this sort are they which creep into houses, and lead captive silly women laden with sins, led away with divers lusts,
7Ever learning, and never able to come to the knowledge of the truth.
8Now as Jannes and Jambres withstood Moses, so do these also resist the truth: men of corrupt minds, reprobate concerning the faith.
9But they shall proceed no further: for their folly shall be manifest unto all men, as their's also was.
Galatians 6:1-10
6 Brethren, if a man be overtaken in a fault, ye which are spiritual, restore such an one in the spirit of meekness; considering thyself, lest thou also be tempted.
2Bear ye one another's burdens, and so fulfil the law of Christ.
1 John 5:16-18
16If any man see his brother sin a sin which is not unto death, he shall ask, and he shall give him life for them that sin not unto death. There is a sin unto death: I do not say that he shall pray for it.
17All unrighteousness is sin: and there is a sin not unto death.
18We know that whosoever is born of God sinneth not; but he that is begotten of God keepeth himself, and that wicked one toucheth him not.
Matthew 18:15-20
15Moreover if thy brother shall trespass against thee, go and tell him his fault between thee and him alone: if he shall hear thee, thou hast gained thy brother.
16But if he will not hear thee, then take with thee one or two more, that in the mouth of two or three witnesses every word may be established.
17And if he shall neglect to hear them, tell it unto the church: but if he neglect to hear the church, let him be unto thee as an heathen man and a publican.
Usually when we meet a true 'false brethren,' and something takes places that puts them on our radar (not that we're seeking iniquity in them, but rather, their iniquity is exposed through their falsehood--a primary example being using the faith the profit monetarily, serving the coin and not our only true God and Savior Jesus), the Holy Spirit equips us with discernment, and He also guides us in how to maneuver the situation (still learning, opening to all correction!).
The manner in which we choose to respond to false brethren, even after they are exposed, could be the very grace they need for the Lord Jesus to be Glorified in God, and in them, which may be cause for them to turn & repent, and become true brethren and remain false no longer.