Beware of two common mental traps that prevent us from "believing" what someone is showing us (I am guilty of both at time!):
When we observe a manifestation of absence, we naturally tend to fill in the void with our own assumptions based on the way WE approach life. We also do the same when we see active behaviors/words that are seemingly inconsistent with our previous assumptions. This projection of thought leads us to confusion when we encounter conflicting information.
If we could abstain from inserting our own ways and assumptions into the situation and objectively observe things, “believing them” the first time would be a much simpler task. Because this is tremenedously difficult (if not impossible) to do, it is sometimes helpful to get outside opinions from trusted sources who will have an easier time being objective from an outsider's vantage point.
Challenge yourself to consider a number of possible narratives regarding your interpretation apart from the one you immediately hold, even if you are skeptical about those other possible narratives. Later, those interpretations can either be accepted or dismissed based on additional observations and evidences.
Secondly, when examining our own lives and the lives of others, we tend to treat both negative and positive traits/incidents as isolated-- to interpret them as exceptional and unimportant, rather than typical and relevant. But once you step back and view things broadly, a lot of clarity is gained. Most traits and behaviors (present or absent) are not exceptional. While some traits are more prominent than others, be assured that whatever is observed is a part of someone’s character, for good or bad.
Our natural desire is to believe that if something only surfaces 5% of the time, for example, that it is not true. However, it is more productive to accept that something observed IS true; but instead of stagnating there, observe to what degree. Keep in mind that even a minor tendency is still a tendency. And tendencies have to be monitored because they typically predict future behavior. It is helpful to know what our own tendencies are and what those of others may be, even if only in a particular set of circumstances.
When in doubt, look for a pattern. Patterns do not lie. But they also take time. Time to reveal themselves. And time to observe. Like a thread running through a seam of fabric, you may only see one piece of it, at any given moment; nonetheless, it is still woven through, even when barely visible.
As a therapist, I often look for patterns. Patterns of thought. Patterns of behavior. Patterns of engagement. Emotional patterns. It has been my experience that I am usually not left empty-handed in my quest.
Oftentimes, we come to concrete conclusions based on patterns and repetition. We do not tend to comprehend things the FIRST time we observe them. Perhaps then, it is more realistic to modify this adage, instead to say “The first time someone shows you who they are, OBSERVE them.” And keep observing them.