As a main healer it kind of sucks to say the only real way to tell is to ask the healers themselves. Majority of the time the tanks can't even tell. It is always the more skilled healer who can tell and unless they are vocal about it no one ever knows really. I will say I alway run with the same tanks and I will tell them if I have issues with other healers coming into party for certain fights. I am at the point though that I simply do not care for 99% of the fight because a healer that simply stays alive and throws out some heals is good enough for the win despite how much harder I might have to work