To answer your original question, how did I know for sure I was trans, honestly, I didn't. I never like to say I'm 100% sure about anything, that's just how I am. I spent a ton of time looking online and in therapy trying to answer that question. Actually, I wanted someone to tell it to me. I didn't trust myself, didn't know myself; didn't know if I was a crossdresser, trans, or just some weirdo who liked to get their jollies off imagining things. I wanted/needed someone smarter than me and someone unbiased to tell me.
I was never feminine growing up, never mistaken for one lol. Tall, hairy, deep voice, big hands/feet. Never saw a girl in the mirror, never imagined I'd ever look like one. Yet, that's what I wanted. I knew that, just didn't trust it I guess?
I never thought I'd ever pass and if that was the case, guess I kinda felt like what was the point? I didn't wanna be seen as a guy in women's clothing. I couldn't handle that myself. I didn't wanna be insulted, laughed at, assaulted, disowned, ostracized, friendless, etc.
However, I was kinda at the point where I felt I had nothing; therefore had nothing to lose by trying, by experimenting. I never really had any close relationships; some friends sure but nobody really close, same with family. Was depressed, hated self, so ya, why not, you know?
I kinda knew what I wanted long ago so I went with it, I went after it. I officially started taking steps towards transition. I kept it private for quite awhile because a. wasn't 100% sure if I was trans or not yet, left me an out to stop if decided to stop later with no one the wiser, my secret safely kept, b. wasn't sure where I'd end up, wasn't sure even where I wanted to end up.
Sure, I thought about it. I thought about it a lot but I never had no set end goal. I never set it in stone that I would go part time, full time, that I would come out to everybody, that'd I'd have this surgery or that. Knew I wanted laser for face, knew I wanted hrt for myself, knew wanted sexy long hair, but that was all. Deal with the rest when it came up, that was the plan. May come up, may not so why worry (if only that was easy to do, lololol). My only real goal was to fix myself, improve myself, be happy with and like myself.
I'd say doing it that way worked for me; taking things slow, step by step, trial and error, experimenting and just letting whatever may come to come, you know? I mean, I worried nonstop about what friends/family/strangers would think. I worried about being alone. Losing everything. Prolly had all the same worries you did. Honestly you can't predict what will happen tho until it does.
I'm not saying jump on the transition train (choo-choo!) or anything but I do think you should do whatever you think would make you happy, cuz that's what matters. Experiment, find out what works and what doesn't for you.