By studying them within the context of the whole Bible. (You are eligible to choose this answer if you have read and thoughtfully considered the entire Bible, while testing specific doctrines.)
By studying selected Bible passages and comparing them with others on the same topic, while testing specific doctrines. (You are eligible to choose this answer if you have looked into these passages for yourself.)
By studying selected Bible passages and judging specific doctrines (without comparing them with other passages).
By reading all or any part of the Bible without testing doctrine at all, maintaining your presumed doctrines, and glossing over difficult passages in the hopes that "we'll understand it someday."
By accepting or rejecting what other people have said about the Bible, be they parents, religious leaders, atheists, historians, friends, or the internet.
You missed one. (Tell me about it in the comments.)