How to know someone is your truly friend/s?

Human beings are very complex I know this sounds very negative but I don't think anyone really knows who is a true friend below the surface, I am speaking from experience here and have seen other cases, I think people are very self absorbed and when it comes to the crunch, very selfish, we all keep growing so we also keep changing, I have had a best friend cheat on me and I have seen other things such as a neighbour's best friend, like a sister, bridesmaid at her wedding run off with her new husband, it is hard to figure human nature out as I see it one should just accept friendship on the surface but always allow for human weakness.