How to know what you believe? I suspect it has something to do with seeing how you react to circumstances or how you behave. Like if you believe fire burns you, you will recoil your hand away from flames. If you believe something you act in accordance to what that belief means. The tricky part might be to know what "acting in accordance" means.
Then I think some beliefs show up as something you just "know". When I ask a born again Christian why they believe what they do, they say they just know and feel the holy spirit is with them. If one is trying to believe in something and wondering if it makes sense to believe it, and then tries to act as if they believe it - that is not going to make you believe it (imho). Actually that's what a lot of religious people are doing. Acting as if, but really don't know. There's a name for that kind of worship that eludes me. But it's like going through the motions without having an inner feeling or knowing about what the motions are. Almost like pretending or doing all the right things just in case the beliefs are true.