How to know if your a good healer? Well it depends on many things.
Reputation is one. When grouping, people that you play with will know if your a good healer by your play style.
While keeping the group alive is the main objective, all experienced players know when things get out of hand (dps pulling aggro several times, adds, etc), no one healer can perform miracles. Those that expect you to perform miracles regardless of their actions (like pulling aggro) are people you don't need to group with again and dont understant healing in the game. They are the people that dont know how to play thier class and like to blame the healer, tanks, whomever, for their subsequent deaths. We have all had experience with these types. Thankfully I am past the levels where I meet alot of these types and most of the people I group with are pros that know what their doing. We can go thru the tuffest instances (poets palace, tables, etc.) with me solo healing with absolutely no deaths. We need to remember that a group must act like TEAM Players. The success of any group is based on the actions of ALL the group members, not just the infamous tank/healer.
Last is your own self assessment. How do you feel your doing? Are you making the most of your abilities? Are you working to obtain your master/adept3 heals and wards/debuffs? And most importantly, do you know your limitations? As the healer it is important that you add your imput on where your group goes to exp. I am very familiar with the group that wants to take on things way over their heads for the "phat" xp (The second they wipe they are placing the blame on the healer/tank). If some things are out of your league, dont be ashamed to say so. Dont get yourself in a situation where you are certain to fail. This isnt a popularity/ego contest. If you cant handle something, let the group know and suggest 1. they add an additional healer to the group or 2. suggest a xp spot that you know you can handle.
Success in healing isnt about doing the impossible, but knowing what is possible. A good healer is only as good as the group they are in. Just my thoughts. Good luck!