Short answer: Generally, I usually register T as an absence of F.
For example, many people mistype T women as Feelers because T women often get good at doing the “soft girly†thing through practice. But as an F myself, I can often spot the lack of true F in such women: I see that there’s still a hard edge there, I see that they don’t “dote†on people or things, I see a lack of softness in how they compete or in their humor or whatever. (I’m not saying that a T’s hard edge is a bad thing; I like it, in fact. Mainly I’m just saying that I register a difference...)
What I said above may seem like an unfair answer to the OP, that is, defining the presence of Ti as the absence of F. But actually I went to the books, and they said pretty much the same thing. Obviously Ti is about logic and objective analysis; but Ti is an introverted function, so the “logic aspect†may not appear in a form that’s obvious to outside observers. So what’s left for the outside observer to see? Well, Ti is so self-contained that the outward features of Ti include calmness, impersonality, imperviousness to criticism or drama, obliviousness to "the human element," etc., in other words a lack of F drama and reactivity. And that gets back to what I was saying at the start of my post...
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