E vs I is pretty easy
If it it isn't obvious because they aren't reserved(I) or extremely chatty(e), the biggest tip off is a need for alone time(I) or constant boredom(e). I keep in mind too that ENxx and ISxx typically come off as about the same extroversion level.
N vs S I have a hard time with
This isn't the best indicator, but the way I try to figure it out is by how easily I can talk to someone. Being an N type I find it difficult to keep up with someone who is constantly talking about what they did last weekend or what they saw or did whenever. I usually find it very uninteresting. N types talk about the same things, but when I throw in an interpretation of whatever we're talking about it usually leads to a whole tangent going deeper into an abstract discussion. S types tend to kind of blow off my interpretations probably because they think it's uninteresting or irrelevant.
T vs F usually easy but not always
Again, not the best indicator and quite biased actually, but F types come off feminine, genuinely nice, and affiliative, and T types come off masculine, independent, and pragmatic.
(There is a correlation between females being F and males being T. Don't assume anything, but it is sometimes a safe base to start off from when trying to figure it out.)
J vs P if they're 'E' than it's usually easy, if they're 'I' good luck
I always try to figure out E/I first because that usually makes this easier. EJ are very strong willed and directing leader types, prioritize on work. EP are very much in the moment, not usually long term planners, love to play. IJ are similar to EJ but toned down. They're usually rigid and have a serious demeanor in my experience. IP are easy going, usually up for anything but sometimes seem lazy.
This is how I try to interpret people, but remember that all people show signs of all other types, so you can never be completely sure.