code: public boolean snapshotExists(String snapshotName)
      {
          assert snapshotName != null;
          for (ColumnFamilyStore cfStore : columnFamilyStores.values())
          {
              if (cfStore.snapshotExists(snapshotName))
                  return true;
          }
          return false;
      }
docs: Check whether snapshots already exists for a given name.
  
  @param snapshotName the user supplied snapshot name
  @return true if the snapshot exists