How do we know who is an antichrist?

1Jo 4:6 We are of God: he that knoweth God heareth us; he who is not of God heareth us not. By this we know the spirit of truth, and the spirit of error.

The spirit of truth HEARS…

HEAR the sayings of Jesus. Matt 7:24; John 12:48
EXAMPLES that match Christ. 1Cor 11:1; Php 3:17; 2Th 3:9; John 13:15; 1Tim 4:12; 1Pet 2:1-6, 21;
APOSTLES Doctrine. Acts 2:42; 1John 1:5; 2:19; John 17:20; 1Thess 2:13; 1Cor 2:9-13
READING is hearing. John 20:30-31; Eph 3:3-4; 1John 2:12-14.

additional outlines from 1John can be found here.