Its a hard problem, to be sure. You will never know what their motivations are unless you work with them. The best that once can do seems to be looking at their work experience, at the kind of stuff and places they've worked on before and then... taking a leap of faith in thinking that the person will do well.
Passion is highly overrated, I think, and a lot of people define it differently. For some, it means staying at the office for 12 hours everyday including saturday. For others, it means keeping regular hours and contributing good quality work everyday, yet recognizing that one might need to work more (without overtime) when the job demands it. And the manager should know not to make these kind of situations a regular thing and reward the employees accordingly.
As to the question of attracting the right kind of people, although it can be faked, the curios ones are usually the best. If the person you're looking at is always asking questions, or making suggestions, not to impress you but to get a genuine understanding of what you do and what you want her to do, that is the best sign of a passionate employee.