Whiskey (sometimes spelt whisky) is produced in many countries all over the world, including Ireland, Scotland, Japan, the U.S. and Canada. Unlike wine, whiskey does not continue to age once bottled, so there is no need to cellar a bottle of whiskey to improve its flavour. Because of its high alcohol content, whiskey rarely expires. However, extremes in temperature, improper storage and other factors can cause the whiskey to deteriorate over time and, in rare cases, become undrinkable.
Look for changes in colour. Over time, whiskey can oxidise, causing both colour changes and changes in flavour. While not usually a sign that the whiskey has gone "bad," oxidised whiskey can lose some of its complexity and become less enjoyable to drink.
Save the date that the bottle was opened on your calendar. The expert opinions differ slightly, but it is generally agreed that a bottle of whiskey should be completely consumed within 1-3 years, with some experts saying that noticeable loss of flavour can occur as early as one month after opening. This flavour loss is due to a higher ratio of air to whiskey in the bottle as the bottle gets closer to empty, so the act of saving the last few fingers of a very good whiskey can actually cause the good whiskey to taste bad more quickly.
Compare the whiskey you have at home with a fresh bottle of the same variety to determine if the whiskey has gone bad or experienced a loss of flavour.
Sealed bottles of whiskey that are stored in very cold temperatures can develop a cloudy appearance. This is not a sign of spoilage, but just a change due to the cold. Always store spirits upright in a cool dark place to prevent premature ageing.