Whiskey is an alcoholic beverage that was first recorded in written history in Scotland, in 1494. Since then, whiskey has become one of the most loved drinks in the world. It is aged inside large casks made from oak, for a minimum of three years. Whiskey aged in casks will continue to gain flavour until it has been aged for about 12 years --- at which point, it stops drawing in flavour from the wood. If you have a bottle of old whiskey, it can be worth quite a lot of money to a collector, or it can make a great conversation piece at your next dinner party.
Check the label of the bottle to see if there is a specific date listed as to when the whiskey was produced.
Identify the shade of the whiskey inside the bottle. The longer the whiskey has to age inside the cask, the darker it will be. Whiskey that has not been aged for very long --- known as Green Whiskey --- is as clear as vodka. The colouring of the alcohol is indicative of its age.
Take the bottle to a nearby off-licence that sells good quality whiskey and ask them for their opinion. If the clerk is trained in whiskey history, he might be able to tell you who manufactured the whiskey and when it was produced.
Taste the whiskey if you aren't trying to sell it to make a profit. Aged whiskey has more time to develop flavour from the oak cast and will therefore have a stronger taste than Green Whiskey. The strength of the flavour is indicative of the age of the whiskey.