By simply doing your research, and understanding the world more and more. Reading especially the website, but other sources as well.
Using good logic to wire everything together, you will understand how what is written in the JoS is true.
You can also see here on the forums what kind of experiences others are having in satanism.
Keeping in mind that most people here wouldn't gain anything if they lied, especially in an anonymous forum, so gaining no money, nor fame, these are genuine testimonials of people and their experiences with the Demons, Gods and meditations.