I think kids should be taught to trust their gut feeling, because that's often right. Not always, but then again, nothing has a 100% guarantee of success.
I don't have kids yet, but looking back on my own childhood, I think an important lesson is to watch people and to analyze their actions. This might help you to see their intentions. Then again some things are just felt, so you must trust your gut. If you get a bad vibe from a stranger, trust it. And I think that's what one should teach children - to trust themselves and their feeling over any seemingly charming words.