When do you know that someone is an alcoholic?

And when would a group of friends decide to stage an intervention?

We have a friend that drinks a lot, but he's normal acting. He can go a few days without drinking and be fine. He can even go out and eat and to a bar etc., and only have a beer or a mixed drink or two. He conducts himself very well, no DUI's or bar fights, etc. But he drinks probably at least a 1/5th a night. We think he drinks at lunch sometimes too because he has to go home for no apparent reason, and he has seemed on occasion a little bit loopy afterwads. (He is divorced, no kids, part time girlfriend.)

I think it's probably mainly out of boredom, but I rode with him the other day to the LQ, and he cruised right by the drive through speaker and pulled up to the window. They opened the window and told him the price, and he gave them exact change. They gave him his stuff, and said see you tomorrow.

He's not hurting anyone or drunk texting etc. or doing anything crazy when he drinks. He doesn't really go out, just stays home and drinks I guess, and apparently a lot.