For a moment I was unconfident in my use of the word; however, after looking up the term, it appears it has many, many different meanings. I will assume you mean the one that says something like;
My question still stands. How can you be sure the realm of human relations in any way fits into a comprehensive 'model' as described above?
In very, very simplified terms, a model is a way of describing something complex in simpler terms. You usually do that by making assumptions that limit the amount of variables you're forced to deal with.
For example Socionics is a form of cybernetics, studying the networking of complex psychological systems (human beings). These constructs can be simplified to perceptual inputs (accepting functions) and social outputs (producing functions). As for why 8 functions, and as for why the rules as to what these functions can be paired with, Augusta and the group she brainstormed with decided on that for whatever reason they did due to what they had observed happening in the real world. Someone else needs to fill in that history lesson though.
Thus we have a model.
To answer your question, how can I be sure? Anything can be modelled. The useful question to ask is, is your model useful? Does it find the balance between wieldiness and accuracy? A bad model either takes too much into account and is rendered inoperable, or it takes too little into account and fails to adequately describe or make predictions about that which it originally sought to model.
More catchy version: a bad model is either a poor simulation or an excellent waste of time.