Nothing ruins a refreshing beer like the presence of an unwanted flavor or odor. Although most beers maintain their quality for long periods, a beer can be damaged by improper storage or by contaminants in the draft lines or brewing equipment. You can't do anything about it, so the best solution is to pour a new one and hope the whole batch wasn't ruined. You can identify a spoiled beer by several telltale signs.
Flat Beer
Not all beers should be highly carbonated. However, when a beer that is supposed to be fizzy lacks bubbles or a head, this usually means that the container has been damaged, allowing the gas in the beer to leak out.
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Skunked or Oxidative Flavors
A beer with an unexpected sour, buttery or papery taste may have been exposed to air, causing a chemical reaction called oxidation. This process alters the chemicals that give the beer its flavor, leading to a strange or unpleasant taste. Beer that has been exposed to direct sunlight can develop a strong unpleasant odor and flavor, or "skunked."
Precipitating Particles
A small amount of haze is normal in some beers, but if a lot of particles are floating in the bottle or glass, your beer has probably gone bad. Cloudiness may mean that the beer was not properly filtered or that some ingredients are precipitating out of the solution.