This is probably the most important aspect when hiring for start-ups. When it comes to technical skills, people can always learn and with time become very knowledgeable, but passion about building awesome stuff needs to be there.
Instead of discussing technical questions to find out if the candidate has competence, I prefer to ask questions about the candidate's opinion around things.
Or a more broad question that tends to get people talking. This is best asked before you talk about the existing stack for your project.
When asking these questions you will quickly realize if someone is passionate about his/her work and can't wait to get started, to have ideas and take initiative. It's easy to spot when someone isn't really that hyped, when they don't know what to say or are being too general in their answers.