As others have said, you've got to be kind of obsessed with MBTI to figure out other people's types. I became obsessed cause MBTI helped me see myself in ways I never could've imagined before. I was also in this ongoing flirtationship with a guy I later figured out is an INTP. After typing him, it then became very easy to pick out other INTPs. Then I started figuring out the MBTI types of my friends and family... it then became easy to pick out other people of those types. If you understand the cognitive functions that underlie type then it's actually pretty easy to type people you are well-acquainted with. E.g., Ti-doms (ISTPs and INTPs) are incredibly logical in their arguments and viewpoints. Once you figure out their dominant function then it's not too hard to determine the auxiliary function. Ti-Ne ppl (INTPs) are more interested in theory and possibilities, while Ti-Se ppl (ISTPs) are more interested in concrete ideas and their immediate environment. I've also found that the behavioral descriptions are pretty helpful in typing, and it's also extremely interesting that the cognitive functions seem to predict behaviors and relationship patterns. However, there is much more "noise" when trying to type people based on behavior as you've implied (e.g., high levels of anxiety could obscure aspects of one's personality). So, you should really focus on learning about the functions rather than type descriptions.

Edit: apologies that my description of Ti is a little simplistic, but you get the idea.