Alcohol - how could we have gotten in trouble?

I live in Montana.

I’m not worried about anything happening right now, because too much time has passed. But I’m curious about the potential ramifications of this situation.

When I was almost 19 (I’m 29 now), I went to a friend’s house to visit with him and his family before we went to our senior graduation party. While we were there, his parents let me have a beer. After a while, we left for our party.

In this scenario, no cops came, nobody got hurt or killed, no accidents happened, no property damage was inflicted, everybody made it to the party and the party went off without a hitch.

What would have needed to happen in order for any of us to get arrested? Could someone have reported us after the fact and gotten us in trouble? Would the cops have had to see it happening? If someone had called the cops say, a few days later, would they have been able to do anything? What if I had been younger and only had a couple of sips of beer rather than a whole bottle?