A friend by any other name?

I’ve recently had an experience with a “friend” that has startled my concept of friendship. We’ve all had some tough times in our lives, I know I have, and I can relate to how difficult ordinary things may seem under these circumstances. My question is regarding why your “friends” may think that it is okay to make you the target of their misery when you have no involvement or even knowledge of the situation. Not only was my experience one of a friend lashing out at me for reasons which at the time were completely unknown to me, it happened it a work setting, which resulted in some extraordinarily unprofessional behaviour and a great stress and frustration for me at work. The later explanation of this behaviour managed to shine some light as to the motivations of the inappropriateness, but the apology was too weak to erase the lasting scars. It also implied that some sort of apology on my end was required. Although I gladly apologize and sympathize with the issues at hand, my words were not heard and my former friend has not spoken to me since. While I realize that this in fact illuminates the fact that we were never really friends, and merely acquaintances, I can’t help but wonder… how can you know which “friends” are your friends?