Use a realistic work sample test. It's marginally more time consuming than a whiteboard interview, but it's fairer and more effective. Most candidates would vastly prefer to commit and document a small patch than solve brainteasers in front of an audience. You'll learn a lot more about their practical skills.
Certainly measurable, but what are you really measuring? If I gave one of my junior devs a serious problem to research, consider and solve and they came back to me 60 minutes later, I wiuld tell them to go jump in a lake...and I'm certainly not looking over their shoulder while they do it. I always say I do my best development in the shower when thinking about a problem fir a while. Unless you plan on jumping in there with me, you'll miss a lot of where my actual problem-solving happens. Short syntax questions to screen and longer-form 2-dayish take home problems for the win... with practically every other approach you're measuring things that just aren't really germane to how good software is developed.
