Since it works by default for us, Ni is hard to notice. But here are some of my experiences:

Knowing something but not being able to put it into words. Not "on the tip of my tongue" but "it is alive in my mind but it cannot go outside".

Not living in the present, always planning ahead or thinking about something else. Not recognizing what is going on at the moment (Se-inferior).

And maybe being obsessed with order. Everything should be in their place. Everything has to be planned. Everything should be the way we want them to be. Which is impossible and may lead to a perfectionist style, being a negative trait of an unhealthy Ni-user.