[8bitsrule]

Step one, it seems to me, is to figure out what passion isn't ... there are several mental trips that may seem like passion, but don't last. I think of it as a challenging journey you're drawn to regardless of what others think (so it's not ego-tripping) or how conventionally rewarding it is (if it is, lucky you, but ... danger, Will Robinson).

Something that opens you up and allows you to develop ... challenging enough to keep you from ever getting bored. A large set of related problems, for example, a universe to apply your growing expertise to.

That (not so elegantly) said, finding it may mean a lot of trial and error. So staying open to new candidates is important, until you can rule out most avenues and focus on those you stay drawn to.