When you have an extra keg of beer left over from a party, it can seem attractive to keep it around for the next one. However, even untapped, the beer in your keg can still go bad. Your keg of beer should have a stamp signifying the expiration date.
The shelf life of beer will vary depending on the brand and whether or not is has been pasteurised. An unpasteurised beer has a life expectancy of 45 to 60 days -- however, once you account for shipping time, this can drop to about a month from the date of purchase. If beer has been pasteurised, it can last three to four months. Most American beer is not pasteurised.
While the flavour will be a sure indicator of whether or not your beer has gone bad, you can look for some other signs before taste-testing. If the beer has gone cloudy or is overly foamy, it may have already gone bad.
For unpasteurised beer it is crucial to keep the keg cool, below 3.33 degrees C; anything above this will cause your beer to foam excessively when dispensed. Because unpasteurised beer contains dormant bacteria, any temperature above 10 degrees C is likely to cause the bacteria to grow and spoil the beer.