Has he come home drunk? Sounds simplistic, but why do you think he is drinking? If he is drinking and managing not to show the signs it sounds like he may be experimenting but doing it fairly sensibly.
As to how you tell...smell the breath. Anything more than this will lead to a huge showdown. But you can buy breathalysers, if you want to go down that road.
Encourage him to talk to you about it, and I dont mean interrogate him about his drinking habits. Talk to him.