I struggled between T & F for 3 years!
We develop our dominant function around age 6
and our auxiliary function around age 12,
so what I did was I went back in my mind to my childhood
and thought about how I was back then.
In simple terms the most outstanding characteristic of a T is that they are tactless.
They are brutally honest, with themselves and others.
Children who a primarily F are diplomatic and care about people's feelings.
As a matter of fact, it was Kiersey's section on children's type in Please Understand Me that helped me with this.
The T child might ask why they have to do something,
where the F child wants to please.
Also, I read the differences between NT and NF.
The Fs I know seem to be natural born diplomats.
They say things in such a nice way, even if it's something kind of critical.
I gave consideration to that idea as well for trying to figure out my preferences, but as mentioned by others in this thread, at least for males in the U.S. F behaviour tends to be discouraged and at best not rewarded so many of us end up adapting and trying to become somethign we are not. I do think looking at our past has a lot of validity in trying to make this analysis, but in doing so its important to ask yourself if what you were doing at that time felt completley natural to you or if you felt you were doing it either to meet expections, try to fit it, or just avoid being criticized for it.
With the exception of imaginary friends, I see myself displaying Ni at an early age and don't see any problem with that stage of my development.
In my teens (and earlier) I also worked on various T type interests (math, computer science etc) while at the same time I was struggling with Fi and Fe related value and emotion issues. During that phase, I think I was simultaneously developing Ti and/or Te (through education and practice of skills) and Fi and/or Fe (through introspection and internal conflicts). The way I see it, I was focused on improving T to adapt to society (the abilities did come naturaly to me and some of them I even enjoyd doing (both then and now)), but I had a natural inclination to F that could not be ignored and I still spent a lot of time focused on intorverted feeling contemplating values and emotions. I can speculate that I would have developed differently if societies values were different, but there is no way to be certain of that.
I have adapted to the environment by supressing my emotions when in public as well as with people that have known me for a long time. I've gradualy got a lot better at accepting my strong Fi side (and moderate Fe), but only show that side of mself with new people I meet in a non-work environment (or ones in a work environment if they decide they want to get past my cold and aloof appearance to get to know me).
Thats just the theory I use to explain my T/F conflict. Even though I think Fi is my dominate trait, I've developed Ti and Te as a result of environment and since Fi, Ni and Te are all traits of INTJ, I sometimes think thats a better fit for me than INFJ or INFP. Even though Fi (which would point to INFP) and Ni (which would point to INFJ) are the traits I feel most comfortable using, my Ne and Fe both feel less developed to me than me Ti and Te. It never realy felt un-natural to do thinking related activities of any kind, but if I look at my core values, they are most certainly NF and not NT.
Here is how I rank my traits accopmapied by how I scored on the cognative processes test. The test felt like it asked too few questions to give it much credibility, but it does match the two aspects I do clearly relate to.
My ranking -- Test ranking
1) Fi -- Fi (47.8)
2) Ni -- Ni (43.0)
3) Ti -- Ne (29.7)
4) Si -- Fe (27.9)
5) Te -- Ti (26.7)
6) Ne -- Te (23.5)
7) Fe -- Si (21.4)
8) Se -- Se (19.8)
This does also point to a pretty clear NF preference, but when I read descriptions for INFP and INFJ both have a lot of elements I relate to and I find I often get along withe NTs better than NFs so I feel I can't completely rule out the idea of being an NT with an overdevloped Fi when neither INFJ or INFP are a flawless fit.