I was maybe about 75% sure when I started HRT. I was pretty sure, at least sure enough that I wanted to try it to see if it really did make me feel better about myself or not, but I'll admit that in the beginning there were a LOT of things from my past that didn't quite fit the official Trans NarrativeTM that made me wonder whether I really was trans or not.
The main thing that gave me doubt was just the fact that I didn't really start feeling gender dysphoria until I was like 13. And although those feelings were VERY strong, to the point that they completely dominated my life and practically destroyed it for a good 5 years, and were still up there despite the fact that I'd learned to suppress them, I was very confused over where they came from in the first place. Because those feelings started at the exact same time that my parents divorced, I moved from Ohio to Florida, and started going through puberty, so there was a LOT of confusion up there in regards to whether I really was experiencing official gender dysphoria, or whether it was just a coping mechanism for the changes I was going through, or possibly just ->-bleeped-<-. I was pretty sure it was dysphoria, just because of how damned long I had been putting up with it, and because these thoughts NEVER went away once they started, but I did still have some doubts. And I actually started HRT with the impetus of believing that a trial run with HRT would help me to sort this confusion out. That I'd see how I felt once I really did start getting all of those features that I had been yearning to have for my entire adult life.
That was dumb. Going on HRT didn't sort out a damned thing in my mind. In fact, it only made it even more complicated because now when I was feeling completely amazing or completely crappy and having doubts, I didn't know whether it was the hormones talking or my rational mind talking.
Going to see a therapist (about 2.5 months into HRT,) is what REALLY helped. That's really what I needed all this time, is just someone to talk about all of this with, and be honest and open about it, which I had never done with anyone before. And it's kind of funny. Now the further and further I'm getting along in HRT, the more and more certain I'm becoming that I'm trans. My therapist really has helped me to sort out where my doubts were coming from. (It was all doubts over how other people perceive me... which has NOTHING to do with whether I am trans or not, it just has to do with social anxiety. And I've realized, on a fundamental level, I have no doubts about what I want whatsoever. Just fears about how others will perceive me once I finally do get a body that I'm comfortable with.)
So yeah... I started out maybe only about 75% sure, now I'm up to about 95% sure. And I'll be honest, I don't think I'll ever be 100% sure about it, at least until I've reached the very end and been living in my final body for a couple of years. Because although most of the time I am completely 100% sure in my head, and most of the time I absolutely LOVE every single thing that is happening to me, there's always still these occasional twinges of fear and doubt where out of nowhere for a few minutes my brain will panic and go "OMG, what the f*** am I doing?" before going right back to being happy with it. So yeah...