"How do you determine someone's type?"
I usually draw a strip of paper out of a hat.
Seriously, I find it a tricky endeavor to try to type someone unless I already know them very well, or unless they come to me wanting to know more about their type and offer enough pertinent information for me to give them a good estimate. Really, the best way would be to have the person take a reliable exam (MBTI, KTS) in a comfortable environment with a knowledgeable administrator, and to chat with them beforehand about how the questions are to be answered (according to natural inclinations and not learned habits, or what they think people want to hear, etc.) in order to promote a more accurate result.
If I do want to figure out someone's type, just for fun or to try to understand them better, and I already know them in more than a casual manner, and it's not practical to sit them down and give them an exam

, it helps to try to start broadly with Keirsey's temperaments. If you can narrow down what their temperament might be, even if you can narrow it down to one of two possible temperaments, that will be quite helpful. Then you may be able to think about how to fill in the missing letters, as it were. Some people are much harder to type than others, even if you've known them for years. I generally regard any typing that I have done unofficially with a grain of salt, as it is possible that I have done it incorrectly, since I am only an observer of any other person.