Is it the behaviour of the grown up children of the Father? The way to tell whether a person is not really a grown up is to see what the person uses to restrain their bad behaviour (social sanction; peer pressure), and what they use to move them to serve God (social approbation; doing what the group approves of).
The grown up children of the Father will look to the scriptures to tell them what to do and believe (rather than the group), and they will look to the Spirit in them both to restrain the bad and move them to the good (not to public opinion). They will be clearly conscious that their only hope of living as grown ups is to trust that Christ is living his life in them.
The sign of a grown up child of the Father is that they live by trusting Christ's life to be lived out in them as he wishes. They depend on his grace in their lives, and they live by the Spirit. This makes them remarkably free. They are free from the opinions of others and so are free to serve others (rather than serving their own reputation or safety). The more grown ups like this there are in a church the more likely the children (of all ages) will grow up in Christ as well.
Dale