Si is remarkable in that it can keep track of everything the person is doing. Ni's talent is figuring out how to do things.
These differences are also useful for distinguishing INTJ and ISTJ. The INTJ sits back and thinks about how to do things, and once having thought enough about it, spends about 1/10 the time doing it as was spent thinking about it. The ISTJ just sits down and starts working on a task, dealing with issues as they come up.
Si doms tend to classify things by what they are and define words specifically. Ni doms tend to classify things by what they do, and use definitions loosely (but arrange their words precisely!). As an easy-to-understand-but-therefore-unrealistic example, an Ni dom will have no trouble referring to anything with a blade as a "knife", regardless of size or brand or intended purpose, while the Si dom will think of each instance/brand/category of knife as its own thing.
It can be more difficult to tell between Si and Ni doms when the Si doms are particularly intelligent/educated, because they'll talk about "abstract ideas", and expound on truths and insights that it would seem only an N type would know. In these cases, it takes time to assess, since one has to wait and see if the individual takes abstract idea A and use it to hypothesize about abstract idea B, or perhaps refine abstract idea A. If abstract idea A stays the same over time, it's Si dom.
Other clues to distinguish Ni doms from Si doms:
- If they say they find it easy to learn languages, they're an Si dom. (Not all Si doms are good at this, but Ni doms generally suck at it, because it involves too many out-of-context associations of ideas.)
- If they go on about how brand X is better than brand Y, or about thinking about buying the latest, fanciest outdoor grill, they're Si dom, no matter how technical their reasons. (Ni doms think in terms of purpose, not kinds of things.)
- If they can regale you with the entire details of their day, they're Si dom.
- If they say the oddest things, and then you realize it's a pun, they're Ni dom. (In general, Si humor is about breaking Si rules. Ni humor is about breaking Ni rules.)
- If they appear to stare for hours into space, yet get everything done that they need to do, they're Ni dom.
- If they always appear to be busy, they're Si dom. (more true for ESxJ than ISxJ, but true nonetheless)
- If they're geeks who play D&D, they're Ni dom.
- If they're geeks who play fantasy football, they're Si dom.