When most of us came to this forum, our types were determined usually by online MBTI tests. Doubting about our types is a natural consequence of the inaccuracy of the tests. But after learning about cognitive functions most of us were capable of answering the question.
Despite of this, sometimes doubts still emerge. I think the cause of this problem is functions not being well understood (which I also have). It is very difficult undertanding the opposite function to our dom (and sometimes aux). And as the same result (behavior, conclusion) could came from different sources (ways of thinking, cognitive functions), confusion is possible.
I am quite sure I'm an INTP and not an INTJ because I'm quite sure I'm a Ti dom. I understand it, I "feel" identified with it, my F function despite of being very low is much more Fe-like than Fi like when emerges, but I still have problems trying to understand how Ni works.
If there is a problem that could be answered by Ti or Ni and I'm not sure how Ni works, or I have a wrong idea about it, how can I be completely sure that I use Ti and Ni? That's the question. In my case, despite of my problems with Ni, all points to Ti and fits with the rest of my functions (Ne, Si, Fe). And I am absolutely sure I have no Te; the clash with my psyche is very strong (I have Ne, the function most annoyed by Te; not an INTJ).
But in people with more "hybrid" behavior, I understand the problems. So the difficulty for this question is understanding completely Ti and Ni from Ti point of view (so an INTP could be sure) and Ni point of view (so an INTJ could be sure)...
Coming back to the main issue, your doubts. Good suggestion have been made previously in the thread. Try to decide if you use Ti as dom or Ni as dom. If not, look at the rest of your functions, and consider the needed balance (Ti with Fe; Te with Fi; if you have Te you cannot have Ne and vice-versa...). A really good point habe been made by @Vel: Ti is "deductive", it works from top to the bottom; whereas Ni is "inductive", it works from the bottom to the top. Maybe if you write what concrete thoughts make you doubting about your type we could help you properly, instead using generalizations.
Don'f forget an option: I remember having read in some thread a post where you said you previously thought you were an ISTP. It you have strong doubts, this could be the solution, because ISTP (Ti>Se>Ni>Fe) uses intuition in the form of Ni (like INTJ's). If you have developed strong intuition, this could be the source of your doubts, you would have both Ti (like INTP's) and Ni (like INTJ's). The solution would came then from your moral system: Fe for INTP and ISTP and Fi for INTJ.