How can you be sure that someone is the devil?

Answer 1

Ask them.

Another Answer:

Actions speak louder than words and the Bible says:

Matthew 7:15-16New King James Version (NKJV) You Will Know Them by Their Fruits15 "Beware of false prophets, who come to you in sheep's clothing, but inwardly they are ravenous wolves. 16 You will know them by their fruits. Do men gather grapes from thornbushes or figs from thistles?

Answer 2

In Christian belief, the devil is a spirit, not a person. Presumably if the devil could impersonate a human person, you could easily tell by the fact that the person had no life history, simply appearing from nowhere, with no known relatives, no friends, no schooling or work history.

More rationally, there is no evidence the devil even exists. There is no mention of Satan (the 'adversary') in any Old Testament scripture written before the Babylonian Exile, and furthermore he is portrayed in these early Jewish writings as the loyal assistant of God, tasked to test the righteousness of the faithful.

Answer 3

If a person conveys evil or immoral stay away from them. their negative energy can affect you in a negaztive manner.

Answer 4

When a person starts to worship the devil, they are not issued with a glow-in-the-dark sign on their body, a secret handshake, or a set of horns. Your best test to date is to ask them.