You know what? Technical prowess matters, but there are much, much easier signs of whether someone will be a great engineer or not. I wish I had the hard numbers to back this up but for now you'll have to take my word for it.
So far, this is the number one indicator I've found. People cancel interviews at the last minute with no explanation or apology. They don't answer their phone when a phone screen is scheduled. They don't respond to email for odd periods of time.
A really good example is a candidate who did not respond at all to his offer letter for about a week. It exploded during that time. When this person finally got back to me, I had to rewrite it with a new date. In all, this behavior was absolutely indicative of work ethic.
In any one of these cases, it's better to err on the side of believing it's an issue and not proceed further.
Example: "Explain to me why you chose this stack to solve this problem?" The person talks and talks around the answer, explaining what these off the shelf components do. Ask again: "I'm familiar with the stack. But what are the requirements that led to this being the best solution?" Again, talks around and around without giving the requirements.
I understand people feel like they need to prove how smart they are. But show me that you can actually hold a 15 minute conversation about some cool work you did, wherein both people can participate equally in said conversation. I ask a lot of "why" questions just to see if I can get people to show an opinion about something and have a conversation.
Follow directions when it comes to solving a whiteboard problem, or whatever. There are lots of examples for this, but here's a really, really basic one. I once had someone who was 2 hours late to the interview because they had gone to an address in South San Francisco instead of San Francisco. Which brings me to my next point:
I'm more than happy to pitch people on our company. One thing I've never understood though is why I'm pitching people about stuff they can easily have looked up before stepping in the door. People are asking me about VC funding as part of the Q/A portion of an interview. That stuff is on Techcrunch. Candidates should be reading this and making decisions before stepping in the door. If you want to ask about it, say something like "I saw So-and-so is an investor, who else is in that round that I haven't heard about and how are they helping you?" Something that at least shows you've done the homework.
Second part is, if this really influences you, please use public info to decide before walking in. Don't ask me to convince you.
For example, when I walked into Groupon to interview, I knew up front that the founders had taken most of a $1bn round off the table for themselves. I had judged that and decided to go forward with interviewing at Groupon anyway. Never asked about it. But while working there, I had to talk about this SO MANY TIMES with candidates at Groupon: "Well what about the founders taking money off the table?" My response was always "I knew about it. Didn't care." Then I actually questioned how much real consideration this person had given to Groupon. Are they serious at all, or just using us for a practice interview?
A couple of times, I've had candidates just shrug at the whiteboard and say "I don't know." They don't want hints, they don't want to talk, they just seem uninterested.
People get stressed, or just hit a mental block when at the whiteboard. I get that. I've had it! I've had it in interviews where I nailed all 7 and then on the 8th had a complete mental block.
But IF you run into that, you'll need to show us something else. Show enthusiasm for the problem. WANT to solve it. Show enthusiasm for the company. Anything! Because if you put on the cap, put down the marker, say "I don't know" and leave it at that, then you might as well just walk out. Which is fine, maybe you have concluded it's not the right fit. But there's no reason to stay if you really don't want to.