How do you know when that one really close friend transitions to being more than a friend?

I have been friends with someone much older than myself for quite a long time, we've always had a close and cuddly friendship, we've even admitted to having crushes on one another. We talk almost every night; we tell each other our problems, cheer one another up and joke around. Our conversations always have some tinge of "what if..." to them, and we both have come to know just how far we can push our joking before it offends the other [though it rarely does]. I don't miss him when we're apart but I think about him and wonder what he's up to. We are not interdependent on one another, we are around one another because we choose to be because we're both happy when we're together.

Some of my other friends have been telling me that he and I might as well date one another already because we "practically are already" hence the question... how do you know when the friendship becomes something else?

Why not just spend more time developing this friendship? Enjoy the time you spend together and let it flow naturally. No reason to rush into anything.

You must spread some Reputation around before giving it to I wish again.

This is how you know, you both admitted to having crushes on each other, but it really seems to me that he is into you but your not if your not missing him when your apart. It almost seems as if he is falling more for you then you are for him. Its best if you set it straight by either telling him your not or if you give it a try you guys seem to really get each other and you could really use a person who gets you plus the time frame for being friends does put a lot of feelings on the table. You get him and he gets you, not why don't you get that it is more than just a friend ship. If you feel that you will loose him completely if it doesn't work its better to know that you tried. Its only a matter of time before that type of friendship will only lead to you guys having feelings towards each other.