The Tank speed on fire is an easy one. We simply go on versus and test if a normal tank can catch a full health runner; then we light him on fire and test the same thing. A stop watch might be handy for this, but I'm not sure how we'll record a watch...
I think I've figured out how we'd do some of the myths.
For the tank's "best shot spot" weak point, we simply have someone spawn as a tank in versus (with cheats) and then blast 'em in different spots with different weapons to see what does what.
For outbreak, we'll need someone who doesn't have the achievement to help us. We simply just throw them in a game with us and see what does what. I'm pretty sure it has something to do with boomers in versus, but I could be wrong.
For hunter/smoker FF, we turn god mode on, then run up to the incapped person, shoot 'em, and see if the character says something like, "Ow! stop shooting me!" Either that or we could find out a way to nullify hunter/smoker damage and then actually record how much damage is done by bullets.
(Gahk, 1000 character limit, gonna have to double post.)