Not everyone who calls themselves a Christian is really a follower of Jesus Christ.
There are numerous groups that refer to themselves as "Christian", yet have startlingly different beliefs.
Practically speaking, Roman Catholic doctrine demands that the current Pope has the final authority on matters of faith. However, no other "Christian" group really cares what the Pope has to say. In fact, some even claim that He is the Antichrist (Westminster Confession 25:6).
Practically speaking, Mormon doctrine demands that the current LDS Prophet has the final authority on matters of faith. However, no other "Christian" group really cares what the LDS Prophet says.
Practically speaking, Jehovah's Witness's doctrine demands that the Watchtower society has the final authority on matters of faith. However, no other "Christian" group really cares what the Watchtower Society has to say.
Protestant doctrine demands that the Bible has the final authority on matters of faith. Interestingly, absolutely EVERY "Christian" group cares what the Bible says.
The Bible tells believers to beware of false prophets, and that we will know false prophets if their prophecies do not come to pass (Deuteronomy 18:22), or if they teach us things contrary to the Bible (Deuteronomy 13). It also informs us that while God used to speak to His people through the prophets, He now speaks to us through His Son, Jesus (Hebrews 1:1-2), and confirms His own words to His believers with the indwelling testimony of the Holy Spirit (John 14:26). Thus, based on the Bible (which all Christians care about), we should not expect that God is going to send a new human prophet to lead His church. We should not be looking for a papal prophet to tell us God's words, an LDS Prophet, or a society of prophets. However, should someone arise claiming to have God's divine direction for humanity, it should be fairly easy to tell if they are on the level. Just see if their prophesies come to pass, or if they ever contradict the Bible.
No two "Christian" groups agree, but generally where they differ, one has departed from scripture. Or maybe both of them have.
This doesn't directly answer the question asked, but with the Bible, the Holy Spirit, and prayer, determining whether or not a particular body of believers is made up of genuine Christians should not be too difficult. Honestly, I myself have yet to find the perfect church.
Remember the Bereans of Acts 17, who were counted more noble than other believers because they investigated the claims of the apostles using the scriptures (the Old Testament) to see if what the apostles were teaching was actually true. You should do the same.
God bless you, guide you, and lead you in your pursuit of fellowship.