How do you know if they're really your friend?

I felt alone whenever I walked home while watching others with someone, but then I managed to meet someone who joined me and let me join their circle. Been two years since that day, I start to question my friendship with them. Even when we walk together, I feel alone. During times of conversation, suddenly everything feels silent whenever I'm around. They're always together, but never bother to find me. Whenever I'm not myself, they never asked if I'm alright. I don't feel like a friend, I just feel like someone who just so happens to tag along. Can someone please tell me how I could really know if they're someone I should keep in my life? I'm scared to lose them, but I don't want to keep hurting myself like this.