code: public static boolean isCrsValid( CoordinateReferenceSystem crs ) {
          if (crs instanceof AbstractSingleCRS) {
              AbstractSingleCRS aCrs = (AbstractSingleCRS) crs;
              Datum datum = aCrs.getDatum();
              ReferenceIdentifier name = datum.getName();
              String code = name.getCode();
              if (code.equalsIgnoreCase("Unknown")) {
                  return false;
              }
          }
          return true;
      }
docs: Checks if a crs is valid, i.e. if it is not a wildcard default one.
  
  @param crs the crs to check.