It doesnt take a rocket scientist (you know, I have never met a rocket scientist) to figure out what someone has a passion for. Just watch them. Give them something to do, and watch what they do. If they love doing the task you gave them, they will still be doing it, perfecting it, embracing it, tweeking it and inviting others to help out with it. If they don’t love it, they will do the mechanical aspects of the assignment so they can get on with what they really like.
If they love the work, there won’t be enough time for them to spend doing it.
If they don’t love the work, ever second they are engaged in the task is a chore.