How to discover mistakes in faith? There are many possibilities how to twist the faith.

For example: assumptions, prejudices, misinterpretation, unintentional deception, projecting wishes... These things can twist the spiritual way, 

How do I find out if someone's faith was twisted?
This is a very valid concern. Here are some suggestions that come to mind.


Study the word regularly. Pray that God would teach it to you and approach it as something that will shape you, not something you need to find support for your cause in. Systematic reading really aides this rather than following along with whatever verses somebody else quotes for whatever cause.
Keep your eye on other traditions. This doesn't mean church hop and float between belief systems, but it does mean keeping up on some reading, listening to some preachers you don't agree with just to make sure the ones you do are really saying something, studying different bible translations to help you get a full grasp of the word.
Stay in fellowship with other believers. Find a community that is both well grounded and diverse. The discussions, interactions and iron sharpening iron is what will keep you on the right track. A community where questioning something and examining the basis for things is not welcomed is not a safe place to be. Likewise a community that does not value truth or place any guards on itself at all is also a dangerous place to be. Use lots of discretion.
Don't be stubborn willed when you learn you're wrong about some little thing. Keep an attitude of humility and and pray regularly about what areas you need to learn and grow in. If God is trying to show you something through any of the things you need to be ready to hear it. Don't be blown around by every wave of teaching that comes along but do keep.


Lastly and most importantly, don't stop looking to Jesus, the founder and perfecter of our faith.