Testing new tanks

I mentioned a few days ago that one of our tanks has moved on from 25 man raiding, which while being his choice, has put us in a bit of a lurch. We're examining several candidates for our 3rd and possibly 4th tank slot, but here is the tough spot.

How do we test them out?

Heroics are easy enough, and once we're at this level of gear they really aren't a test of skill. I suppose we could have them run some undergeared alts through, but that isn't a real test. In all honesty, the tank could be Kungen reincarnated - but if they can't work with the team we have, we won't restructure just to get them.

Establishing goals

It's unfair to test someone when you don't have an established set of guidelines and goals. Here's what we (or I) am looking for when I test a new tank. Really this can go for any player, but this post is about tanks so we'll go with that.

1. Can they follow directions? We're not looking for a new raid leader - I've got that covered thank you very much. We don't need a screamer or someone who get's pissy - I need someone who can tank what they're supposed to, where they're supposed to.
2. Can they not taunt when they aren't supposed to? Nothing gets my goat like a tank who taunts off another tank - just because he wants the glory. If you're taunting because there's debuffs, 42 adds, whatever - that's fine. But don't taunt adds or bosses just because you think you're the magical MT.
3. Can they hit their CDs? If a tank dies, the first thing I look at is my raid cooldown timer and recount/GrimReaper. I want to know what happened, and if there was any way to prevent it. If his Last Stand/IBF wasn't burned, or my LoH is still available, we have work we can do.
4. Don't be that guy. You are not the greatest tank since Kungen (I only assume he's great - I really wouldn't know), nor are you the greatest tank I've ever seen. If you think you're going to waltz in and take over, this probably isn't the place for you. Yes, tanking requires a certain level of play - but some of the best tanks I've ever raided with never said a word outside of organizing pulls.
5. Interview well. It's going to happen. Don't tell me "I don't need any tank gear from Ulduar because I've got all 216/219 anyway". Awesome. Let me know how Freya +3 works for you in your other guild.

I think for now we're going to take new prospects to Sarth 3D and throw them to the wolves. There's no tank swapping, but that fight involves being at your best the whole time. One 'ooops' and you've wiped the raid. We might try an achievment run in Naxx to pick off those that we missed pre 3.1 - but Sarth 3D should test them in the right ways.

How do you test new tanks? What can I do different?