I was raised in such a situation. I was very devoted to it for many years, however, there were always questions I had about certain doctrines I was taught that never stacked up against what I read in scripture. I was told that those were verses we wouldn't understand this side of glory, and/or, that I needed the Holy Spirit to help me interpret them their way. So, I started looking around. I found a lot of groups that called themselves Christian and didn't agree on doctrine. I found that to be unacceptable.
I always assumed that the bible wouldn't lie when it said that our God isn't a God of confusion. Funny thing is, many of these groups have no trouble saying the others are Christian so long as they all leave the others to their own thing. What they absolutely can't abide though, is when a group claims to be Christ's Holy Church, because that upsets the apple cart. Nobody likes to think that they're believing in modern innovation and the doctrines of men.
It boils down to someone having to be wrong. Some groups are more or less wrong than others, but that doesn't make them any less wrong. There is one Truth and one faith. There isn't such a thing as agreeing to disagree. The Truth often seems arrogant to those who don't believe it. It makes people feel judged when all that is being asserted is, 'This is THE truth. You either adhere to this faith or you do not.' There's no personal judging going on as one has the choice to follow the truth or not.
Anyway, how do you know? Well, if it is a false gospel it will eventually lose it's luster, because it will ultimately not produce any good fruit in you. For me, I went to church every week and got absolutely nothing out of it. So, with the lack of truth around, I eventually left.