How do you know if you are best friends with someone?
A friend is usually someone who you see occasionally, and maybe hang out once in a while. A friend can also be someone who you put a little trust into. A friend is just above an acquaintance, who is someone you know by name and few little facts about.
And a best friend is usually someone who you spend A LOT of time with. Also, a best friend is someone who you confidently put most of, if not all, your trust into.
You can usually tell if your someone's best friend if he or she goes to you with his or her problems and ask you to help them. Or if they personally ask you to hang out (but this has to happen numerous times before the person is considered your best friend). Basically, you know if your friend puts most of his or her trust into you.
I hope this helps ^_^