Jesus warned us that “false Christs and false prophets” will come and will attempt to deceive even God’s elect (Matthew 24:23-27; see also 2 Peter 3:3 and Jude 17-18). The best way to guard yourself against falsehood and false teachers is to know the truth. To spot a counterfeit, study the real thing. Any believer who “correctly handles the word of truth” (2 Timothy 2:15) and who makes a careful study of the Bible can identify false doctrine. For example, a believer who has read the activities of the Father, Son, and Holy Spirit in Matthew 3:16-17 will immediately question any doctrine that denies the Trinity. Therefore, step one is to study the Bible and judge all teaching by what the Scripture says.
Read more: http://www.gotquestions.org/false-teachers.html