No matter how diligent a person is, there’s always a chance that someone malicious could slip something into your drink on a night out. Even an innocent non-alcoholic beverage can ruin your night and perhaps your life if spiked. "Date rape drugs," have led to at least 200,000 rapes in the US alone according to 2007 Department of Justice statistics.
One of the tell-tale signs of a spiked drink is a blue tint, which is presumably easier to spot in a glass of Chardonnay than a Blue Lagoon cocktail, but there are available test strips for sale. Advice from Wired on how to tell if your drink has been spiked with Rohypnol, GHB, ketamine (roofies): Check Your Drink.