While I don't have a test to give you, there are a few points that can be helpful in distinguishing these two.
In Jung's work, intuition is abstract perception and sensation is concrete perception. The essential difference between these two is that intuition always goes beyond the details of experience and perceives abstractions derived from experience (patterns, connection, possibilities, all of that stuff usually attributed to intuition), and sensation focuses on drinking in the experience and absorbing every minute detail.
That's the problem, it depends on my mood - I do both quite often.
The terms abstract and concrete can be a bit misleading with respect to introverted sensation, though, because concrete implies 'realness' or earthiness, and while Si-preferring types are indeed earthy, the nature of this function is such that their focus is on the concrete perceptions of the inner experience. Jung spend more than a few words describing the introverted sensate's focus on bodily experience, but their realm of perception extends beyond their physical form; any sort of inner event (bodily or otherwise) is subject to Si's perceptions. If a person becomes suddenly sullen, the ISxJ will find themselves arrested by the mood, attempting to absorb as much of it as possible. Compare this to the INxJ, whose focus is much less on the mood itself than it is diving the impetus of the mood or the possible actions they might take as a result of the mood. This of course doesn't mean that INxJs won't wallow in their moods and ISxJs won't try to figure out what's wrong with them, but that in general, all other things being equal, this is the general frame of thought that each type holds.
Based on all of this, the best way to define Ni and Si (if we are aiming to be close to Jung, that is), would be "abstract perception via the inner world" for Ni (which echoes Jung's definition of 'perception via the unconscious') and "concrete perception via the inner world" for Si. One common misconception about the introverted irrational functions is that their sole focus is on perceiving the inner world, and, while this is not untrue, it does forget that the orientation of a function does not limit its machination solely to that world, but instead provides the frame of reference through which the function processes information. For example, introverted thinking is introverted, but it is entirely capable of evaluating the external reality. The difference between Ti and, say, Te, then, is not what it evaluates (inner/outer reality), but from where its standard of evaluation comes. The same is true for the perceiving functions. Ni and Si are not limited solely to perception of the inner reality, but their perceptions will always be referred back to the inner world, so as to further the introverted goal of venturing deeper into the psyche. The concept of impressions is a good illustration of this dynamic: the Pi functions perceive the impact external reality has on the psyche, what the outer world releases in the subject. Of course, like any introverted function, their preference will always be to stay within the inner reality (since interacting with the outer reality takes some elbow grease).
If these functions perceive by the inner world, that begs the question of what that means. Jung spent a great deal of lip on the introverted functions' relationship to the images of the collective unconscious (hence the 'perception via the unconscious' from earlier). The most prevalent example of this relationship is with Ni; much of its perception are located around these primordial images and the abstract perception thereof (if you can image abstraction abstractions further)--it perceives the possibilities and patterns within the unconscious, which is where Ni gets its propensity for symbolism and metaphor. It is my estimation that Si's relationship to the unconscious is a little less distinctive; these images are by nature elusive and difficult to grasp, which goes against Si's focus on the concrete aspects of inner experience. While I'm sure there is a relationship, I imagine that ISxJs are only tangentially aware of it (as an Ni-dom I can attest to the fact that even I am only somewhat aware of Ni's unconscious machinations).
I do have unconscious 'knowings' sometimes - I often 'just know' something's going to happen.
The case for Si:
I have a very sharp fact-based memory. Like, I still remember everyone's birthday from a group that we used to be active in, that knowledge doesn't even do me any good but I still feel like the facts are 'stuck in there'.
The case for Ni:
I do often know what will happen next just from the mood portrayed 'between the lines'. I also hear that Ni-Users are always 'trying to improve - even at the cost of security and tradition', which perfectly describes me. I believe concepts of respect and tradition are subjective and even dumb, sometimes.
Any sort of attempt to define the cognitive processes is somewhat futile. We might apprehend some aspects of their nature, but they are a lot larger than mere processes that can be reduced to x + y = z. The functions are frames of mind, fields of consciousness, whatever you want to call it. They are best defined by their limits, thus any definition can only be partial. Of course, if the definitions of the functions were even slightly clear, I doubt we'd still be discussing it today. The human psyche is a complicated thing, and this is only one tool in understanding it. I hope this has helped you out.
Agreed. Too many people take Jung's theory as gospel. It's possible for someone to be both a strong Te AND Fe user (not me, but I've seen MANY people who are).
