Some trial and error... But critically... I am 30 now and I believe I am doing my passion. I hit an "inflection point." that point had some key parts... I got intensely focused for several weeks, studied like crazy, to earn a position at google. I didn't get in, though they are constantly recruiting me.. Strange indeed. But... That focus and subsequent denial caused me to consider my future path in life... Very, very seriously... And I decided that in fact all life experiences of all people are NOT the same. I wanted a better life for myself despite my rejection, and I wanted to be happy on that path. No more bullshit anymore. And so by stripping away all my future path options that consisted of varying degrees of bullshit, I was left with paths that constituted things I was passionate about. Things I really cared about.
And so the problem reduced to me instantly growing up and seeing all the nonsense around me. And opting out of it.
If you are having problems finding your passion, you either need more different life experiences to push you one way or another... Or you need to be more critical of the experiences you have already had.
Finally, once I realized what it was that I was "passionate" about, I committed to it, prioritized it very highly and focused on it. And I had no problem doing that. It made me happier to do that.. That was the positive feedback I needed to confirm that in fact it was my passion. ;)