An ENFJ friend told me that he figures out whatever a friend is really worth being kept as friends by testing them at the beginning of a relationship. For example: he once tested a person by giving them money for absolutely no reason. When the person took it without hesitation, he conclude that the person was greedy and not worth being friends with.
I find that this is a very interesting and useful way to figure out someone. Have you nice people in the NF idyllic find yourself testing a potential friend by deliberately doing something and then observing their reaction? What qualities other than greed are you testing for? Any other ways to figure out a person other than giving out money for no reason?