Fi. I'm in the exact same position as you. Nobody ever types you as INFP except for an INFP, it is weird. Don't know about I or E. I'm really lucky I initially typed myself as INFJ, because if I typed myself as INTJ or INTP, I probably would be an INTP right now being really really insecure about my type like this person is doing. I do it every few weeks. It is not fun. Slowly making my argument for INFP stronger though.
I like how everyone isn't answering his question about Fi. It annoys me.
INFPs can type INFPs pretty accurately. We remain uncertain though, and I will remain uncertain, but I currently think this guy is INFP. Curious thing. I'm always uncertain, yet my first instinct is to say I know. Only error I ever catch 100% of the time. Just saying, because of the ambiguity of Fi, and the uncertainty of Ne, this leads to doubt. Every few days, I think I could be an INTP or an INFJ or an INTJ. These are ridiculous, but still. My dad is an INTP, and we are EXTREMELY similar. BUT. We are pretty different. He can quickly take apart systems such as cars and stuff. Not I. Never.
How to tell if you are INFP. Does math bore you? Do you not get technical things quickly? Do you understand people better than cars or computers? If neither then someone is wrong somewhere. Probably me with my generalizations of both types. Good luck in typing yourself, I'm going to go help some other person be typed.
As for INFP in its natural form, see Ludwig Van Beethoven. Yes he was a REALLY REALLY good musician. BUT he was trained to be one from birth. He became very skeptical of people. I find that interesting, because I am very skeptical of people. Also Edgar Allan Poe. I feel as though I am 30% Beethoven, and 30% Poe and the rest would be what confuses you into thinking I'm INTP. Hope this helps, but I'm sure it only served to make myself confused about what type I am.
You know what. I've decided you are an ENTP. Case closed. I just typed several, and you sound like them. Don't ask why, it would require me going back and forth between them, and I don't want to do that.