Identify how people interact with you, and gauge who they are from there. If you want, you can even develop a system for yourself to subtly tease out a person's type, like for instance, if you want to know who a senser is, ask them if they have seen any movies recently. If you have seen the movie, pretend you haven't, either way begin asking them to describe what it was like. If they give you a lot of descriptions relating to the five senses, then it is likely they are sensing dominant. Want to know if someone is a judger or perceiver? Find out how long it takes them to complete a task, and in which way they structure things they have to do. Make a joke about randomly trailing off and doing something else, while you were supposed to be focused on a certain, different task. If they laugh or identify with that, they're most likely a perceiver. These are just a few ways, and once you become more familiar with it you can sense it pretty easily.

It helps to know about how the different aspects can be introverted and extroverted as well, and also how certain behaviors from their upbringing can appear to be like another function. It isn't to tell if someone is an S or a J when they're organized if they were brought up by a really harsh judger or senser themselves. They probably engrained some of their own personality into them. Life experiences can do this too.

But it's not like the structural integrity of a building is depending on the accuracy of this, it's mostly just fun for some people to guess. If they find out they are wrong, because they committed to one type they are more likely to remember why they were wrong when they find out what type the person actually is - if they ever do.