This is a zombie thread but I'm new here and there's not much going on, so I'll add my .$02.
Unless you've quit being a parent..wouldn't that be nice sometimes?... regardless of their age, if you have a concern about something they're doing, you bring it up with them. That's what parents do. You don't sleuth the answer or try to read their mind or ask 20 questions.
The real issue is how exactly to do that without shutting things down. That's the tricky part, and we'd need more info. For instance, with a minor, you'd open the conversation about whatever the concern is from your perspective, you talk more about you than them...in this case, drinking..you'd them know exactly where you stand on that and what your expectation is...including the other parent. You don't question it nor do you present it in a provocative way, and you surely don't dance around it based on what everyone else does. You reiterate what hopefully you've been teaching about your family and it's values.
If they're older, it's similar, but your leverage isn't so much based on your authority, but on the bond that you've hopefully established by the time you need to use it. Adults are entitled to make their own choices, even if we don't approve, but likewise as family members, we are equally entitled to share our perceptions and concerns..as well as our desire.