It may not be obvious but this question is the same as “How do we know Jesus is true and not a liar, or confused?’ The exclusiveness of Christ’s claims, as noticed by any Bible reader, make him either the proverbial ‘liar/blasphemer, lunatic or the only Christ’. If he is the Christ then he is the only Christ, only ultimate prophet, only ultimate way, only God, only ultimate truth:
I am the way the truth and the life. No one comes to the Father except through me. (NIV John 14:6)
Standing from a point of unbelief the question is much different and as an outsider to faith the answer would be that we certainly do not know and can not know while we remain in darkness. From the standpoint of faith in Christ, however, the Bible affirms that we can know. Luke wrote his gospel partially ‘so that you may know the certainty of the things’ we have been taught.(NIV Luke 1:4). Jesus also confirmed that we might know. To those who accused Jesus as the liar and blasphemer Jesus said:
Do not believe me unless I do the works of my Father. But if I do them even though you do not believe me, believe the works, that you may know and understand that the Father is in me, and I in the Father. (NIV John 10:37-38).
John was concerned that people know with certainty the things Christians believe so he wrote 1 John partly to help people discern all the liars about that tried to twist the faith to arrive at some other truth, rather then the Christian one.
We know that the Son of God has come and has given us understanding, so that we may know him who is true. And we are in him who is true by being in his Son Jesus Christ. He is the true God and eternal life.(NIV 1 John 5:20)
All other religions reject the claim that Jesus was the Jewish Messiah who atoned for the sin of the world, calling him a liar or a lunatic. Even within the title of Christianity others say one needs to believe in somebody else other than Jesus only, to be a ‘true’ believer, such as brother Joe or Brother Bob. However the Apostle Paul even refuses himself or angels from interfering with Christ’s exclusive claims:
But even if we or an angel from heaven should preach a gospel other than the one we preached to you, let them be under God’s curse! (NIV Galatians 1:8)
We can know for certain if we are a Christian living in the light, but if we are not then we know nothing spiritually speaking and certainty is only a dream as we walk around in darkness, lying to one another and hating one another. (1 John 1:6, 2:9, 2:11, Titus 3:3).
As far as denominations go that's a side issue to the real question. It simply does not matter in knowing 'the truth' or not knowing 'the truth'. This is because the Truth is a person, Jesus Christ. Therefore, so long as our faith is in the only High Prophet, High Priest, Messiah and Lord, who is Jesus Christ, and that such faith is placed solely in Christ's work on the cross for the forgives of our sins, we can certainly know the truth as the Bible declares.