There are a lot of ways to try to identify who your "real friends" are. You might think they are the people who always remember to wish you a happy birthday, would bail you out of jail at 2:00 am, or are the most fun to get drunk with. All of those criteria, and many others, have some merits and some flaws. However, I'd like to nominate a different definition. With real friends, you both have the sense that you are taking more from the relationship than you are getting. There exists a beautiful non-zero sum symmtery where each of you gain more than you put in.
Common wisdom holds that healthy relationships are all about equality but is this really true? Think of relationships in which you feel completely equal with the other party: they may have given you something at some point and you returned the favor. You may have been wronged but some subsequent act of generosity then righted things. Everything is perfect... Or rather perfectly fine. To me this is the definition of a transactional relationship and is the terminal point for the vast majority of relationships. There is some subconscious tally of where both parties stand and at the end of the day when all is reconciled, both parties are merely satisfied. Deeper relationships on the other hand always feel a little unfair.
Most "unfair" relationships however, don't end up lasting too long. Professional or personal, relationships that are too one-sided eventually lead to annoyance and then fizzle out unspectacularly, well-intentioned or not. I see it all the time: a seasoned career vet with heart full takes on a mentorship role, only to realize that it's not all that energizing to coach someone on how to get promoted every 2 years. Even the brightest and most extroverted career superstar will leave every meeting feeling a little more drained, having given more than they received. Unfortunately, the net "taker" in the relationship often goes unaware, leading to some perplexing eventual heartbreak. While these relationships most often tend to be toxic, on rare occasions they evolve into what is ultimately, real friendship.
Real friendship occurs when both parties walk away feeling like they got the best of it. You think about the relationship and have a deep sense of gratitude; after all, these are the people who have suffered your insufferability. Listened to your emotional outpourings, patted your back as you yakked out of the Uber, and inspired you when you felt lost. So when the time comes to lend an ear or a dollar, you don't even think twice. You know the scales won't balance but you're happy to do as much as you can in gratitude. And on the rare occasion where you muster up the courage to express that gratitude, you are slightly surprised and warmed to have that gratitude met in-kind.
While this definition, stated eloquently by Bill Lazier and restated by Jim Collins, seems awfully selfish at first, I think it captures the true essence of what it feels like to be in a strong relationship. This definition extends beyond time spent together, frequency of interaction, or even type of relationship. And scoffs at the notion that great relationships are "equal" in the colloquial sense. The only tricky step is asking whether the other party feels the same inequality. When in doubt, make up your own reality where everyone loves you infinitely — I can't say it works forever, but I can say that it's worked for 27 years and counting.