In most every instance, the serum IS NOT contaminated, it is fine or flakey precipitate. This is not an uncommon occurrence and most FBS will contain small amounts upon thawing. How quickly the blood is processed into raw serum usually dictates the amount of precipitate, the longer the amount of time allowed for the red cells to clot, the less precipitates. This is because the fibrin is not completely clotted out and even though the raw serum it is filtered during final processing it remains in solution. When the serum is frozen and then thawed, the fibrin can come out of solution giving rise to fine or flakey precipitates. If you do believe that your serum is contaminated, try plating it out or putting some in media and incubating it. DO NOT put a thawed bottle of serum directly into an incubator. Always add it to cell culture media first.