Although a lot of people do know which beers and wines are vegetarian, or stick to a couple they know are, I think it is good to have a comprehensive understanding and list. Alcohol can contain many things which make it not vegetarian, such as: gelatin, isinglass and casein.
As a vegetarian I have found Veggie Wines a really helpful tool. It tells you all about what can make alcohol not vegetarian and then also has up-to-date listings of popular and supermarket brand beers, wines and spirits and whether or not they are vegetarian. It is a fantastic resource, so get revising your vegetarian alcohol so that you don't get caught out down the pub!