I know this has been addressed before in various threads in various ways, but it's something that still puzzles me.
The Intuitive/Sensor split tends to be that which I doubt most about typing other people (though I don't overall have great faith in my ability to type others.) This can be the case even with someone I've known a very long time. A lot of people aren't interested in MBTI and even if they've tested or self-typed, I often don't find that very definitive or reliable (and often they don't either.)
For instance, I had concluded that one of my friends was either ISFP or ISTP...she then took an online test and got INTJ. I asked her if she thought it was accurate and she said yes, in some ways at least, but she thought that she'd also answered some questions in terms of what she wanted to be, not what she actually was. (I could particularly see that as her scientist father is a probable INTJ.) She laughs at me for over-analyzing...I just don't see how she could be INTJ.
I really haven't got a clue how so many people do the sort of "So I was at a bar, and this INTP guy I'd never seen before hit on me." You'd never seen him before and don't know him - how do you know he's INTP?!
Anyway...I find other preferences easier to discern, I think, but N vs S can be very difficult, except perhaps when someone is quite extreme. (For instance, my flatmate doesn't seem to have the slightest interest in or inclination toward any type of abstract thought, or at least discussion; I'd have a very hard time seeing her as an N type for that reason, among others.)
I'm not asking for a definitive "this is how you tell" directive. I'd just like to know how you all personally determine the difference, to the best of your ability. (I really don't buy the sort of "the eyes have it" approaches, but if that's how you do it, I'd be interested to know anyway!)