I put dots on everything at the start of pulls and see how low the tank's health can get before they need heals. Sometimes the answer is "they die before I switch off cleric stance because they don't use cooldowns." Last weekend a tank died three times before we got to the first boss. He quit and I was happy to drop bc the dps (and tank) were all incredibly bad. Archer pulling, both dps not waiting for tank and taking hate, tank not stunning, tank completely mucking up the tough corner pull with the two maids. Two out of three tank deaths were 100% not my fault. (The other one I was partially to blame but dude, would it kill you to use a cooldown OH WAIT.)