You technically can’t know for sure – only God knows someone’s heart. However
““Beware of false prophets who come disguised as harmless sheep but are really vicious wolves. You can identify them by their fruit, that is, by the way they act… A good tree produces good fruit, and a bad tree produces bad fruit…Yes, just as you can identify a tree by its fruit, so you can identify people by their actions.” Matthew 7:15-17, 20
And these fruits are
“But the Holy Spirit produces this kind of fruit in our lives: love, joy, peace, patience, kindness, goodness, faithfulness, gentleness, and self-control. There is no law against these things!” Galatians 5:22-23
As opposed to
“When you follow the desires of your sinful nature, the results are very clear: sexual immorality, impurity, lustful pleasures, idolatry, sorcery, hostility, quarreling, jealousy, outbursts of anger, selfish ambition, dissension, division, envy, drunkenness, wild parties, and other sins like these. Let me tell you again, as I have before, that anyone living that sort of life will not inherit the Kingdom of God.” Galatians 5:19-21
As well as someone that at least acknowledges they believe the Gospel. But even so, we all mess up. “I don’t really understand myself, for I want to do what is right, but I don’t do it. Instead, I do what I hate. But if I know that what I am doing is wrong, this shows that I agree that the law is good.” Romans 7:15-16. And even further, not everyone purposefully living a sinful life can automatically be deemed as a false believer –
“Dear brothers and sisters, when I was with you I couldn’t talk to you as I would to spiritual people. I had to talk as though you belonged to this world or as though you were infants in Christ. I had to feed you with milk, not with solid food, because you weren’t ready for anything stronger. And you still aren’t ready, for you are still controlled by your sinful nature. You are jealous of one another and quarrel with each other. Doesn’t that prove you are controlled by your sinful nature? Aren’t you living like people of the world?” 1 Corinthians 3:1-3
Notice they’re still in Christ, just infants and are still being controlled by their sinful nature without even realizing it.
Personally I believe we should give the “infant in Christ” benefit of the doubt especially online. For some people though if they’ve lived certain ways for years with no remorse or are just so far off there’s no way they believe a thing people say in my head I will assume they’re a false believer and if they’re trying to teach others a false Gospel, warn against it.
All copyrights for this article are reserved to Christianity