If you can take them around the site, perhaps as a second stage once you've weeded out the obviously unsuitable candidates, then it's a good opportunity to see what sort of questions they ask, what they focus on, what gets their interest etc, and can also get them talking in a slightly more relaxed way than a formal interview setting.
Experience is useful but IMO a willingness and ability to learn is more useful - I want people who pick up new things rather than the "I was taught it this way and it's the only way I'll do it" type.
Similarly perhaps see if you can get them to debate a few points as you go. Sycophants are of very little use as you want people who will both push and support you as you'll do for them, but if you hire people who are so full of their own opinion that they'll forever be reluctant to go with your final decision once they've had chance to debate it then they will be hard work further down the line.
Assuming they've all got a CV that checks out and have the relevant experience / intellect to do the role, I'd focus on personality and how they interact with you and other people you meet around the site.
I've been to interviews where they've set formal "you have 10 minutes to prepare a presentation to the senior team on how you'd handle issue x" scenarios, been given all sorts of pointless numeracy/literacy tests, done "influence assessment" team exercises and all manner of other silly things at interviews and honestly I'd just focus on talking to people, try to get them relaxed (if they don't relax then they may not cope with the more high-stress aspects of the job), and get a feel for how they see things, how quickly their brain works, and whether you'd want to work with them (and would trust them etc).
Basic things like presentation matter, too, IMO. If you can't be bothered to iron a shirt, clean/polish your shoes, and put on a suit (or equivalent) then it's an instant no. If you're a 16 year old student wanting to work in McD's for a few hours a week then it's understandable, but you'll potentially see this right the way up to recruiting for QA managers...
Be aware you'll probably get it wrong a few times over your career, and hire someone who seemed great but turned out to be a bit of a nightmare (yep, voice of experience 
), or potentially pick someone because the pool of candidates isn't that great and you need to fill vacancies. Sometimes it goes the other way, and the "I'll have to settle for the least-worst" actually turns out to be a fantastic staff member. Or maybe I'm just terrible at interviewing and should start rolling dice to pick new staff? 
I agree with Ryan M in general - it's hard to find good people!
