This article was originally published on Becky Davidson's blog. It has been modified and republished here with permission.

How do you know when your teenage son's or daughter's friend is a good fit? How do you know when a friend of theirs is really a friend? As a mom of teens for nearly 10 years now, I've had many opportunities to observe the various friends who've crossed my kids' paths. Here are three things I've observed about genuine friends.

1. A good friend will bring out the best in your child

In other words, your son or daughter will tend to be his or her best self around that friend, the effect sometimes lingering even after said friend has left. How do you measure this? It's observable! If your daughter, for example, seems lighter, brighter, happier, kinder (especially to siblings!), more connected to her dreams and gifts and sense of humor, and generally to all the hopeful possibilities of her life, then that friend is a good one.

2. A good friend will never undermine your parental authority or love

He or she will not talk negatively to your teen about your family's culture, rules, expectations, values or anything else you hold dear. It's that simple. Nor does a good friend use manipulation as a lever to get your teen to do anything that could be viewed as a rejection of family beliefs or infrastructure. If a friend respects your family and what it stands for, then that friend is a good one.

3. A good friend - either intuitively or consciously - strives to practice "compassionate joy."

The concept was initially Buddhist, but translates beautifully to any world view; the idea being that if your child succeeds, then the friend, too, desires to celebrate that success rather than resenting it or being envious of it. When life blesses your child, a good friend will feel delighted, not threatened. The friend capable of feeling compassionate joy is a good one.

If we're using these guidelines to thin-slice our teens' friends, then we likewise ought to be actively encouraging our teenage sons and daughters to be that friend to others — working to bring out the best in their friends; respecting the family values of which their friends are a part; and being sincerely overjoyed when their friends' lives take a brilliant turn.

I've watched my kids collide with all kinds of friends, and I've seen the results. Naturally, I nourish a particular affection for the friends who have proved over time to be an especially good fit.