10. Ask yourself it she can do anything about it.

Should you say something if your friend a book club has a stain on her sweater? Should you drop a hint it up if your mother-in-law has spinach in her teeth?

I heard a great guideline years ago that I use frequently. If there is something the person can do about it, you should tell them. If there's nothing they can do about it in that moment, you probably shouldn't bring it up.

So if your best bud has spinach in her teeth, she could fix it with a quick trip to the Ladies' room, so you should mention it to her. When you run into your neighbor at your kindergartners' class party and you notice she has two different shoes on, don't say anything because there's nothing she can do about it in that moment. She'll be embarrassed without anyway to remedy the situation.