One of the keys is the ability to share openly. If we are with someone and we must constantly watch what we say or self-censor too much, then a real friendship is unlikely. Sharing both joys and sorrows is critical to a meaningful friendship. That can't happen if communication isn't open and expressive. We shouldn't be hesitant to open up to a real friend.
Be willing to try and experience new things together is a good test of friendship. When any of us leave our comfort zone there is some tension and nervousness present. Even something as simple as trying a different cuisine because your friend likes it can reveal a lot about the state of the relationship. Traveling can be another test. Being together for several days while away from the security of home and routine may strain a budding friendship. If you can laugh together at misfortunes and share great experiences, then a deep friendship may be possible.
A core of common beliefs and the acceptance of different convictions must both be present. While these points might seem contradictory, I believe they are critical. Common beliefs might include the importance of respect for other people, that discrimination has no place in our society, or that children deserve the very best we can provide. Different beliefs may be about spirituality or religion, political affiliations, and hot-button issues of the day. Friendship requires that those differences are never used as a wedge or weapon. Spirited discussions and honest disagreements should create a stronger bond between two people who value that relationship.
There must be no pressure to "perform." Think back to a dating relationship you have had. Small talk and overt politeness are part of that world. We want to present our best possible face to the other person. But, in a true friendship, it is perfectly OK for one person to be having a bad day and admit it. We don't have to always look or feel our best. That isn't real life and friends don't want someone to put up a front or play a part. "Dress-up" isn't part of this type of relationship.
There must be a sincere interest in learning more about that person. Nothing could be more unfulfilling than to spend time with someone over a long period of time and never learn more about each other. That would mean one or both people are being dishonest about their feelings and needs. It would mean that the relationship would never become more than skin-deep.
Many people much wiser than I have made the point that friendship brings depth and joy to someone's life. True friendship is a special gift that two people give to each other. As Proverbs notes, "Disregarding another person's faults preserves love." Deep friendship is an important part of a life lived well and fully. Maybe it is not too late for me!