Unfornately, there is no method to do this in Windows Server AppFabric (neither in Azure Cache).
The best you can do is an extension method like this one
public static bool Contains(this DataCache dataCache, string key)
{
return dataCache.Get(key) != null;
}
You have to pay the Serialization Tax which can be problematic. It depends on you context.
The only workaround I know is by using regions : CreateRegion returns false if the region already exists. Never tested how it scales, but I don't think so.