Well, realizing you're trans at 26 is kind of late to come to that point, though compared to someone who knew since they were 7 or so it's pretty late in the game.
Looking back though, here's what I've got:
1. I always had more satisfying orgasms when I didn't touch my own penis.
2. I often drank to allow myself to become more feminine, and was nearly a full-blown alcoholic when I finally quit.
3. SEVERE body image problems, wishing desperately to be more fem/andro, which led to eating disorders.
4. Never feeling fully socially integrated as a man, even if I liked (and still enjoy) some traditionally masculine things.
But the one telltale sign I had, more than anything, was a generalized dysphoria that made every day of my life harder and harder to live. I couldn't concentrate, my sleep schedule was out of whack, my moods were all over the place, tiny things would get to me, and I began slowly losing control of my life. That's what people forget: gender dysphoria can manifest as a diffuse dysphoria with no apparent cause. I was afraid I might have everything from schizophrenia to borderline personality disorder, and I was terrified that I would transition and it would end up being a huge mistake.
Luckily, things have only gotten better from there, though on the climb back up I've had plenty of time to realize just how far I've fallen.
Basically, I had the classic downward spiral of a late-onset case, though not as late as some (some girls make it to their 40s or 50s but I don't think I would have survived that long).