One very easy tool I’ve found for self-investigation is questioning my beliefs. It wasn’t enough to have beliefs but to understand why I believed such-and-such, what motivated me? And do I really believe it or is it something that I’ve inherited from the world around me? What are really my beliefs?
I call it easy because it’s a way of looking at yourself without really looking at yourself (it has the appearance of “questioning and investigating belief systems”, but it’s an interesting preliminary step to finding our own motivations behind how we think). Our beliefs are symptoms of the way we think, and so too what ideas we find most appealing or convenient for explaining the world around us.
I’ve also learnt that beliefs don’t represent static statements on the nature of reality (at least not for me) but are simply ideas we have about the world that need testing and updating every-so-often, because as our understanding expands, and experience and reasonings develop, so should our beliefs, aligning more with “what is” rather than “what could be”.