You probably shouldn't even have to ask a passionate person this; at some point in the interview, they probably would've told you what they enjoy doing outside

You probably shouldn't even have to ask a passionate person this; at some point in the interview, they probably would've told you what they enjoy doing outside of work. Regardless, there's no perfect way to interview someone for a development job. It's equal parts experience, your feeling about them, how they fit into the existing organization, and a ton of other variables.