The part where Verbal says "I killed Keaton." Before realizing his mistake and saying "Keyser Soze killed Keaton." And the part where Verbal yells "Oh beep When the detective asks him who Keyser Soze is, like he wasn't even going to mention him in his story at all. And the part where Verbal doesn't say anything until the detective tells Verbal that he is smarter than him, then Verbal looks like he takes it as a challenge and begins to tell his story. Those are all clues I have seen upon multiple viewings of the movie.

i noticed we never see him get arrested, not to mention the aloof amount of confidence.
If you want to be taken seriously, you have to be a serious person.

I watched the film for the first time in 2004, and it was easy to guess that Verbal was Soze after years of hearing the movie had a twist.
On rewatch though, there's the serious way Verbal reads the card during the lineup, the way he's fixated on the various things scattered on the board before Kujan walks in, the way he reads the bottom of the cup that Kujan is drinking out of, etc.