First and foremost, you should always be able to be yourself around this person. No matter what roles you may play with others, what pretenses you put on for whatever reason, all these should fall away when you're in the company of a treasured friend. They should accept you for who you are, and never expect you to be anything else just to please them.
No matter how long it's been since you've seen them, when you get together it should feel as if no time had passed. You pick up your friendship as if you had seen them yesterday. The passage of 50 years does not matter in a treasured friendship.
They stand up for you, even when they believe you are wrong. They may quietly pull you aside and tell you privately what an a**hole you're being, but only later, after supporting you. (This is a hard quality to find, and is one of the things that differentiates between a friend and a treasured friend.)
Finally, if you need help of any kind, they are always the first to call. They don't ask how you got yourself into such a mess, they say: "What can I do?" Sometimes they don't even ask; they know what will help and they just do it. It doesn't matter what's going on in their own life, they make time for you. A treasured friend walks in when the rest of the world walks out.
If you're lucky, you may have a handful of friends like this. Cherish them, for although they know you're far from perfect, they love you anyhow.