The Scripture gives many warnings about false teachers,
Here are only 4 references:
The apostle Paul warns the Galatians:
Galatians 5:7-12 Ye did run well; who did hinder you that ye should not obey the truth? This persuasion cometh not of him that calleth you. A little leaven leaveneth the whole lump. I have confidence in you through the Lord, that ye will be none otherwise minded: but he that troubleth you shall bear his judgment, whosoever he be. And I, brethren, if I yet preach circumcision, why do I yet suffer persecution? then is the offence of the cross ceased. I would they were even cut off which trouble you.
Then near the end of his life, Pau warns Timothy:
2 Timothy 3:1-10 This know also, that in the last days perilous times shall come. For men shall be lovers of their own selves, covetous, boasters, proud, blasphemers, disobedient to parents, unthankful, unholy, Without natural affection, trucebreakers, false accusers, incontinent, fierce, despisers of those that are good, Traitors, heady, highminded, lovers of pleasures more than lovers of God;Having a form of godliness, but denying the power thereof: from such turn away. For of this sort are they which creep into houses, and lead captive silly women laden with sins, led away with divers lusts, Ever learning, and never able to come to the knowledge of the truth. Now as Jannes and Jambres withstood Moses, so do these also resist the truth: men of corrupt minds, reprobate concerning the faith. But they shall proceed no further: for their folly shall be manifest unto all men, as their’s also was. But thou hast fully known my doctrine, manner of life, purpose, faith, longsuffering, charity, patience, Persecutions, afflictions, which came unto me at Antioch, at Iconium, at Lystra; what persecutions I endured: but out of them all the Lord delivered me. Yea, and all that will live godly in Christ Jesus shall suffer persecution.
The apostle Peter warns believing Jews:
2 Peter 2:1-3 But there were false prophets also among the people, even as there shall be false teachers among you, who privily shall bring in damnable heresies, even denying the Lord that bought them, and bring upon themselves swift destruction. And many shall follow their pernicious ways; by reason of whom the way of truth shall be evil spoken of. And through covetousness shall they with feigned words make merchandise of you: whose judgment now of a long time lingereth not, and their damnation slumbereth not.
Jude also warns believing Jews of false teachers:
Jude 3-4 Beloved, when I gave all diligence to write unto you of the common salvation, it was needful for me to write unto you, and exhort you that ye should earnestly contend for the faith which was once delivered unto the saints. 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.
How can we identify false teachers?
2 TIMOTHY 2:15 Study to shew thyself approved unto God, a workman that needeth not to be ashamed, rightly dividing the word of truth.
Compare Scripture with Scripture.
Who said (wrote) it and to whom was it spoken (written) to?
How does it apply (to me) today?