How do I know if my child is into drugs?

How do I know if my child is into drugs or just moody? 

Sri Sri Ravi Shankar: That’s why it is important for you to know them and their friend circle. You will get to know what they do, where they go and keep a tab on them, especially when they are in their teenage.
Teenage is a very difficult time. Teenagers are undergoing so many biological changes and so they are not just satisfied with the love of their parents. They look for love from outside – through emotional support or physical pleasure. Their mind is in such confusion. So, to deal with teenage children is a big task.