A central part of Greco-Roman philosophy, which was rooted in shamanism, was the use of critical thinking (elenchos) to distinguish base magic (trance, which leaves everything to chance, and may lead its practitioners to consort with falsity and evil daemons), and higher magic or theurgy. The latter is a guarantor of truth and happiness, combined as it is with the source itself: the Good, the True and the Beautiful.
Besides the Hara and Heart meditation, it was critical thinking that rescued myself from madness. As a result I came to believe that Socrates represented an ancient path of wisdom, where philosophy in a similar way was used as a navigator through the Spiritual Twilight Zone…

A quite central thing is to be openminded to fallibility (unfortunately this is also abused in New Age, which accuses any doubters and critics of being closedminded). But to be openminded to fallibility is to be open for that both yourself and the guru could be wrong or even false. The concept of enlightenment is often characterized by a religious idea that an enlightened person contains almost the same qualities as God himself. Remember Shutan´s words about that even if you have opened your spiritual heart, and experienced a glimpse of enlightenment, you are still an imperfect human being. Any true enlightened person is characterized by humility, and will acknowledge own failures. Here you ought to be aware of thought distortions such as Selective thinking and Confirmation bias. Therefore: be aware of self-contradictions and hypocrisy in the person. These precisely arise due to a lack of acknowledging own imperfection.