As Christians we should be aiming to have biblical doctrine. Unfortunately a lot of people are using some really faulty methods to work out what that is. These methods lead many to follow all sorts of false teachers and doctrines.
Here's a few of the ones that i see people rely on too often
Sincerity/ niceness- While there is probably some fraudsters who deliberately fake sincerity, most preachers are truly sincere about what they teach. The problem is it is possible to be sincere and wrong. Nor does being the nicest person alive protect one from teaching error.
Good deeds- I have often heard the good deeds someone does used as a rebuttal against questions about doctrine. Any one of any persuasion can do good. It doesn't tell us if their doctrine is correct.
It sounded right- Sometimes stuff sounds right because it lines up with how you've been conditioned by culture, not because it lines up with the bible. It may sound right because you'd really like it to be right (eg. God wants you to be super rich!). Clever writing can also go a long way to making something sound right which actually is not.
I brought the book in a Christian bookstore- It would be nice if buying a book in a Christian bookstore provided a guarantee of theological soundness. Not the case unfortunately. Lots of books with bad theology make their way into Christian bookstores. Some of them even become best sellers.
So what should we do? Compare the teaching to the bible. Not just in a superficial grab a few out of context verses way but in careful and prayerful study.