I make a mental list of their traits, which paints an overall picture that I compare to the MBTI profiles. That usually narrows it down to a few types. Then I consider functions.
If a person is particularly hard, I write the traits down, which somehow clarifies it further. Then I take it to a forum and ask for opinions, if I still need help. If I'm not close to someone, I am often content to just figure out their temperament. I find tests to be unreliable (I mean, they often type me incorrectly...), especially that stupid facebook one.
And yes, this is just for fun, but it also sheds light on our interactions and gives me ideas on how to improve communication and stuff like that. I also start to spot trends in types, and that further increases my understanding of that type and how its functions work. You get better at it with practice, basically.