Whether evaluating a candidate for a potential fit or comparing two very different but competent engineers, I have found there are two traits that help me separate the wheat and the chaff.
I prefer the folly of enthusiasm to the indifference of wisdom.
-- Anatole France
First off, you can't fake enthusiasm. There is simply no substitute for the intangible value that enthusiasm supplies. Someone who is consistently happy and has only average ability is worth far more than someone who is very skilled but always grumpy or disgruntled.
If you are not fired with enthusiasm, you will be fired with enthusiasm.
-- Vince Lombardi
Ask yourself if you trust that they'll get things done if (and when) you leave them alone. Find out if they're looking to get on your specific bus, or if they are just looking for a ride, any ride. Make sure they can demonstrate passion for both positive and negative things around them. Even the most staid among us can get passionate about things we love or enjoy. But if they are broadly accepting of all annoyances, they probably aren't going to be very creative about working efficiently. If they won't talk about the little things that bother and motivate them, they probably lack that insistence on quality that true performers must have. Likewise, they should be able to articulate why the things they enjoy are enjoyable! If you can't get pin them on both extremes, rest assured their work product won't be either.
Creativity is a natural extension of our enthusiasm.
-- Earl Nightingale
The second trait is a follow on to the beauty of passion: inquisitiveness. Given opportunity, do they ask lots of questions? People with passion want to understand problems and propose improvements. The natural state of an evolving mind is to be filled with questions. Smart people, experienced people, know that questions are an imperative prerequisite to understanding. If they don't want to ask questions, they aren't really seeking to understand.
Curiosity is, in great and generous minds, the first passion and the last.
-- Samuel Johnson
When you observe how someone questions, you see how their mind works. You see the speed with which they assimilate and incorporate new information. You often will see the clues to their biases in the solutions they espouse; you can see how disciplined they are by how they stay on focus or by how rigorously they apply new vocabulary. When questions don't have answers or answers that are incomplete, you can glimpse how they handle frustration, disappointment, and even their own ignorance. One of the quickest and surest ways to gather insight into the mind of another is to watch them learn something new.
Curiosity is idle only to those who fail to realize that it may be a very rare and indispensable thing.
-- James Harvey Robinson