HM Mando Raiders: Tell the tank he is not allowed to AoE taunt with no straight line pulls. Basically he/she can single target taunt, but must LoS all pulls. Gets them in the habit of controlling packs and therefor controlling incoming damage (plus works on aggro building)
I already do that but with saber throw.
When ever looking at tanking prospects I've always found it best to simply take them on a few hardmode flashpoints and rock the boat a little. It might make you look like a total scrub and make the recruit question how good your guild really is. But their reaction and how they deal with the issues will be a great indicator to the quality of tank you're looking at.
It's also a good idea to bring a mix of heavily and lightly geared dps and be a little short on the heals. Keep an eye on things like interrupts, threat distribution, movement and ability usage (CC and threat generation).
If you're looking to improve the performance of your current tanks, it's a good idea to first identify the areas they're weak at. Solo challenges and extreme dps conditions might not be too effective if the tank is playing a clunky class like say the guardian. Each tanking class has a strong point and a week point and thus can't be held to the same standard.
But in general I'd apply the hard mode flash point test to identify the weak spots of the tank, unless they are otherwise known. From there I'd make suggestions on ways the tank can improve their game play. I'd also consider rotating the tanks around different jobs in raids.
My old WoW guild never had a main tank, instead they had a group of tanks that would take turns at tanking different bosses. Only during certain fights where a specific class had a clear advantage or we were really short on time would we consider going with X tank over Y tank.