I'm assuming upper and lowercase is in reference to the Socionics tests? Anywho, Internet tests of all kinds tend to be pretty inaccurate. If I only paid attention to test results, I would've solidly been an INTP, which wouldn't really have made sense. What really helped me understand MBTI were the cognitive functions.
Warning: this is going to be long, but I'm putting it up here instead of PM'ing it in case anyone else comes across this thread and was curious.
People primarily use four functions, with the first one being the dominant function, the second being the auxiliary function, the third being the tertiary function, and the last one being the inferior function.
The dominant function describes the person's natural approach to understanding the world. This is important, because someone who seems quieter can technically be an extrovert, while someone who seems louder can technically be an introvert, or someone who seems like an INTP can actually be an INFP. So not all ENFPs will be bouncy, perky, and giggly, and not all INTJs will be serious, cold, and science-y. It's a tendency of the majority, so you can still stereotype each MBTI type and come up with some accuracy, but it makes people who don't exactly fit the description difficult to type.
The auxiliary function describes how that person then translates that information. This is what helps differentiate an INFP from an ENFP and an ESTJ from an ISTJ. They may seem somewhat similar on the outside, but their thought processes vary because of the order of the functions. An ENFP will approach an issue by coming up with different possibilities and then narrowing them down based on their values, while an INFP will use their beliefs as a guide for the sort of possibilities they will explore.
The tertiary and inferior functions are areas in which a person lacks development. It's kind of like the anti-thesis to the first two functions, or what the person's naturally good at. For example, ENFPs are great at coming up with ideas, but tend to suck at following through because they tend to suck at building a plan of action and lack the patience for doing things in small increments rather than all at once. That can be explained with cognitive functions.
So when I type people, I pay attention to what a person's good at and what they're bad at. What they excel at usually builds the first function, and what they're bad at helps confirm their first function. It sounds overly simplistic, but this can be anything from the way they dress (do they care about how they look? is it stylistic? minimalist? lack of attention to detail? what does that tell me about their personality? what does that entail?) to the way they speak (what do they enjoy talking about? do they ramble or do they know what they're talking about? how are people reacting? what does that tell me about the personalities of the people around them and what do i know about relationships between these specific types?).
Once I have a lead on their personality, that narrows their type down a whole lot. It's totally fluid and it can change when I learn more about them, but I tend to have a general idea of how they are and build it around that.