four cards

Look at the four cards below.

We know for the fact that each card has the person's age on one side and what they are drinking on the other.

You are working as the batman (a step down from health minister, admittedly, but probably a step up from roulette-machine checker).

Anyway, suddenly the manager comes in, looking worried.

He's had a tip-off that the police are about to pay a visit, and wants you to double-check that everyone who is drinking alcohol is over eighteen.

Of course, you don't want to bother more customers than is absolutely necessary.

So how few people can you get away with checking? And which ones?