Start paying attention to whether that person shows concern when they see you troubled and if they offer to help or support you in any way they can. “A friend in need is a friend indeed” may sound like a clichéd statement, but it is a fact that one simply cannot deny. A casual friend or an acquaintance may show a bit of concern when they see you worried, but you would hardly find them going out of their way to do something about it. Only a good friend would be willing to do that.
A good friend is someone who is interested in listening to what you have to say. Your thoughts and opinions matter to them and therefore they pay attention when you express them. Notice if a person consistently pays close attention when you are speaking. The person does not necessarily have to agree with what you say; the mere fact that they are listening goes to prove that they care about what you have to say.
Observe how often this certain person asks you for help. If they come to you too often to ask for a favour, then there may be a very good chance that they are just using you and interested in extracting benefits from their acquaintance with you. You must, however, keep in mind that a person may come to you for help regularly if they think of you as a really good friend and have faith in your reliability.
A good friend does not wait for a reason to meet you or give you a call. Texting you at least once every hour to keep you updated on their life and themselves on yours becomes second-nature to them. Note if one of your friends has been doing that for some time now because if they are, then you have got yourself a really good friend.
If a person regularly goes out of their way to do something for you such as planning a surprise party, getting your favourite food from across the town, etc., then he/she is definitely a very good friend of yours and you should hold on to them tightly.