Will an addict do whatever it takes to hide their habbit?

I believe that a friend of mine has been using Meth again. He says that he hasn’t in over 3 months, however, sometimes he sleeps for exessive amounts of time and doesn’t answer his cell phone and uses silly excuse that he was out of town and in another city… or when he calls sick in to work and says he has the flu and doesn’t answer the door or the phone. I think he is using again. How can I get him to admit this to me? Or how do I truly know that he is using again?