I don’t have an answer for your question about how you might become confident that something really doesn’t exist (other than a generic ‘reason well about social behaviour in general, taking all possible failure modes into account’). However, I would point out that the example you give is about your group of friends in particular, which is a very different case from society at large. Shapeshifting lizardmen are almost certainly not evenly distributed across friendship groups such that every group of a certain size has one, but rather clumped together as we would expect due to homophily.
Edit: I see this point was already addressed in Bezzi’s response on filter bubbles.
I don’t have an answer for your question about how you might become confident that something really doesn’t exist (other than a generic ‘reason well about social behaviour in general, taking all possible failure modes into account’). However, I would point out that the example you give is about your group of friends in particular, which is a very different case from society at large. Shapeshifting lizardmen are almost certainly not evenly distributed across friendship groups such that every group of a certain size has one, but rather clumped together as we would expect due to homophily.
Edit: I see this point was already addressed in Bezzi’s response on filter bubbles.