I unconsciously type people all the time through behavior instead of cognitive process. You can't type cognitive process because you can't know why someone does something unless they tell you. Two people of different types can take the exact same action. It's they why that lets you know if someone took an action because of Fe vs Fi.
However, primary and secondary cognitive processes leads to a systemic preference in behavior, behavior that the MBTI has assigned letters to. So I analyze the patterns of unconscious behavior to make determinations what type they are.
S and N is the absolutely easiest to figure out. I can tell through conversation by they way they organize their thoughts.
I find that E or I is the hardest to determine. It's hard to tell if someone is Social Introvert vs an Extrovert. And ENFPs have a tendency to need alone time but for different reasons than Introverts so it's a tricky letter to figure.