The way it works usually for me, is I will usually know something instantly, it will come to me and in that moment, I couldn't explain to you how I know. However, given time I can retroactively determine (usually) what things around me, or from in my mind that caused that initial insight to come to me. It's kind of a backwards process, but it somehow works out. I have thoughts, and then have to retroactively determine what thoughts were. I actually rather hate that I think that way sometimes as I fear it can make me come across as a charlatan. As such, I have put a lot of effort into being able to tease apart the nebulous abstracts that go through my head. It's a never ending process for me. I have gotten good enough that sometimes I can get them to run in parallel. For as amazing as Ni can be, it can also go horribly horribly wrong, in particular if it's trusted too implicitly. Naturally, I seek to avoid that. It thus makes me seem more Si-like because I train myself to be that way. For these reasons, if I didn't have a background on MBTI, I'd guess I am either Si or Se based off this.
Ultimately, I think people may have combinations of these, or this is how it initially starts.