You can tell by the way they run around in the dungeon too. They don't actually follow anyone if they are lost, the one in totorak got turned around cause of how maze like that place is, and had an obvious client hack since it died and didn't respawn at the start of the dungeon.
I have yet to see a healer that acts this way, but with the gil sellers promising fast leveling, its pretty bad when you get one. I doubt you'll see any in post 50 dungeons, unless the gil sellers start promising fast myth grinds :/
The latest one i had in tam tara didn't ever even wait for us, he would pass the boss line and run straight up to the priests. It was too mindless for even a bad tank.
Even bad tanks will talk to you if you say something to them, or they will respond by at least changing tactics.