The best way to help your child distinguish between truth and myth in relation to other cultures is by explaining to them about stereotypes and how cultural prejudice creates some lies about different cultures.
Explanation:
If your child tells a lie about a particular culture, it is ideal that you show him or her what information is true and explain to him or her what cultural differences are. After that, it is correct that you explain to your child how stereotypes and cultural prejudice create false information about the different cultures of the world. It is also important that you show how cultural differences are a plus in humanity and how we should respect all cultures different from ours and not spread false information about them.
If your child comes home telling you something that is not true, tell them about what is the truth in a nice way. Don't straight of tell them that they are wrong. You might start of telling them " Really? But did you know the real truth in this matter is...." or " I think you might have heard a misconception of this. In truth in what i believe in, it is..."