Most likely you can't tell, and fortunately the battle is God's. Fortunately people with addictions have to come to terms with letting go of that crutch, and learning healthier ways to cope with anxiety, and stress in their lives.
You can only go by there word, and often times they hide it, and use it, and other times they don't. It is and up and down battle, and takes time. The best thing you can do is go to a c-dependents group, or alanon. Or Some other group to give you support and feedback to get through it with him.
The other thing you can do is pray over him daily with good intentions.