Attitude - you can teach skills, you can coach behaviour, but attitude is very hard to change. I'm looking for someone who can work on the team; who wants their peers to succeed (not just themselves). People who champion ideas; who encourage people to grow and support them when they're struggling; who share their knowledge... they make a massive difference.
Passion - it's hard to get much out of someone who doesn't give a crap. This doesn't mean they work stupid hours pulling hero efforts, it's about enjoying what they do and having a desire to do it well. It doesn't have to be their first or only passion in life, but they should care.
Curiosity - some people call it the hacker gene, it can show as habitual learning and experimentation. Basically I want to see some evidence that they are curious about how things work, what comes next, how they can build things with it, etc.
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In a more literal sense, how do I decide if they're good at a particular skill? I try to ask them open questions that get them to tell me what they do and don't know. I'll have them talk through their approach to solving a problem. I'll dig into specific areas the job absolutely requires and make sure they talk confidently about working with that tech.