Jesus gave two important teachings that can help anyone know if a pretender of goodness is actually the antichrist.
1. A kingdom divided against itself cannot stand.
In Mark 3:23-26 Jesus tells people:
…23And He called them to Himself and began speaking to them in parables, "How can Satan cast out Satan? 24"If a kingdom is divided against itself, that kingdom cannot stand. 25"If a house is divided against itself, that house will not be able to stand. 26"If Satan has risen up against himself and is divided, he cannot stand, but he is finished!…
Thus at the end an evil person cannot do good, he/she can pretend but by Jesus teaching if we examine people's actions against the yardstick of Jesus' call for loving one's enemies, praying for those who persecute us, blessing those who curse us we can ascertain if a person is or not pretending to be good. Satan cannot be humble no matter how hard he might try to disguise himself.
2. By the fruits. Jesus said in Matthew 7:15-17 Jesus teach his disciples that: 15"Beware of the false prophets, who come to you in sheep's clothing, but inwardly are ravenous wolves. 16"You will know them by their fruits. Grapes are not gathered from thorn bushes nor figs from thistles, are they? 17"So every good tree bears good fruit, but the bad tree bears bad fruit.…
This second statement parallels the first one.
In conclusion: hate, resentment, arrogance, jealousy, cannot disguise as love, humbleness, service to those who practice the Christian values.