I have a rather specific method. Usually someone will do something that gives me some guess as to what type they might be. Usually it has to do with their relationship to me. If it's relative to me, it's usually a guess as to quadra. Then my next step is all about intertype relations. Who are they particularly close to? Do they surround themselves with alphas, betas, gammas, deltas? Do they complain about people who are clearly one type or another. Who are they most themselves around? For instance, I work with a guy who is very plainly NT and a woman who I type as some kind of feeler. Now, I had no idea that this woman was as gregarious and interesting and friendly and funny as she is, because when I first met her she was around this NT dude and she was totally withdrawing into something that was not herself. That's a common reaction to polr interactions. Then, this NT dude keeps talking about logical problem solving, and working in a very Ti (as opposed to Te) fashion. So then I begin to suspect he's an LII, and that accordingly, she's probably some sort of gamma or delta sensor, given her really painful reaction to Ti. Given that I suspect she's a feeler, I now guess SEE or ESI for her type (and the more I think about it, the more one of those makes a lot of sense for her. She comes off as an introvert, creative subtype, so I guess ESI-Se is my current type assumption for her).

I also think that I tend to type in terms of the IEs that I understand intuitively. I can spot an Fe-leading type from 100 paces, with pretty reliable accuracy (they frequently remind me of actors, and I know PLENTY of actors). I can also discern certain types of Ni-leading folks pretty quickly as well---IEIs tend to be pretty easy for me to spot because there's something I recognize in their attitudes/detachment. (And now that I think of it, relative to a debate that seems to have recurred on this forum, there do seem to be relatively few people I'd type as IEI in my general interactions. I don't even know that I'm irl friends with anyone else I'd type IEI. But maybe that's because I want to be a special snowflake, I dunno). I don't understand Si and I tend to overtype people as ESE (perhaps as a consequence of that) so I try to be very very careful about that typing. Honestly I just try to be careful about typing anyone as ESE or LSE because those are types that I tend to get wrong.

If I decide to type someone who I don't already have a hunch about, I generally start either with Club (NF, NT, ST, SF) or Temperament (IP, IJ, EP, EJ). Oddly enough for intuitives I tend to go for club first, and for sensors I tend to go for temperament first. Then I'll refine Club and/or Temperament into a decent guess or three about type.

Above all, I try to come up with a well-supported guess about a person's type and then behave towards that person as-if they were that type, and then see what happens. I look at their interactions as if they were the type I guessed they were and then see if they follow the theory or not. For instance, LII co-worker is secretly quite chatty and loves to have long, energetic conversations with our mutual boss (who is clearly also some sort of Fe-valuer). But when I enter those conversations I immediately flow better in them (get more attention, more laughs, more agreement, etc.) than he does, because while he values Fe, I am, well, better at it. So that's another point of evidence in favor of the typing.