Friendship???

How do you know if someone is really your friend and has your best interest at heart? I know that I am a very giving person and try to make sure that I stay constant in my friends lives and check in on them... If they need something from me I do my best to get it, but I have never felt used or taking advantge of.. However there have ben some people in and out of my life that I know were not true friends. It took me the longest time to realize it but I am glad that I have and they are gone...