Kind of hard sometimes, I usually just feel it out holistically.
Biggest difference is that S's tend to focus on concrete detail -- the FACTS -- while N's constantly are thinking about the insinuations and implications of the facts as their native tongue.
And that's what makes it hard. It's not that S-heavy and N-heavy people don't overlap, they do. You have to tease out their foundational view, however.
S people can talk about implications... but usually they talk about them as certainties... based on their past experiences. They are very aware of implications they have EXPERIENCED and tend to talk in those very real terms. (This is where you get the image of the ISTJ engineer who is adept at explaining exactly what will go wrong; it's not that theory necessarily says it's true, it's that he has experienced it going wrong and thus expects it to.)
N people can talk about facts, especially if they are trying to make something work in the external world, but they are much more at ease at thinking in terms of implications as the base language and tend to weigh possibilities with more nuance. There's a big difference in this with NT vs NF, which makes it kind of hard to describe (N has more flex in its expression than S). To N's, theory/abstract perception IS reality, and they can speak naturally in that mental algebra.
An interesting overlap area is religion. I know a lot of female ISFJs who are religious, for example, in the conservative realm. You would think 'faith' would be more an 'N' thing, but it's not necessarily; however, it is implemented differently. I find N's far more flexible in discussing faith issues; N's think in terms of possibilities; the S people tend to take religious faith and make it "real facts" and then follow it as if it were proven science (since, after all, there is just ONE reality... which is the one the S perceives; multiple realities do not tend to make sense and/or are confusing for S's to work through). Thus they speak in faith terminology but the S's have an underlying perspective of "this is real experience" and make it concrete and real... N's are more more apt to approach it on a mysterious, ambiguous, conceptually defined level.