I can think of quite a few examples, as it's happened to me so many times in my life...
But a common example, in my opinion, is if the "friend" in question, is if they don't seem interested in you at all, as a person. Or say mean things to you constantly & make fun of you.
I also feel like, a person who isn't a good friend, is someone who really doesn't listen to you. Like, at all.
There used to be these girls who lived here who acted like they were my friends (they weren't), but were constantly trying to make me "cool", and called me names constantly, called me lame, were mean to me, and amongst other things. One time I went to their apartment for a sleepover (mistake), and the older girl started kicking and pinching me, god what a bitch. Eventually, I realized they were never really my friends, and were just bullies, and one time wrote nasty things about them in a notebook, and it felt good. I don't know why they started acting like this to me, but it's probably because they didn't like my interests, or me. Needless to say, they moved away, and I'm glad I never have to see them again.