@Override
    public Object process(Node nd, Stack<Node> stack, NodeProcessorCtx procCtx, Object... nodeOutputs)
        throws SemanticException {

      Operator<?> op = (Operator<?>) nd;
      checkOperator(op);
      String opKey = op.getOperatorId();
      if (opMap.containsKey(opKey)) {
        throw new RuntimeException("operator id reuse found: " + opKey);
      }
      opMap.put(opKey, op);
      return null;
    }
Checks some operator quality rules.

Checks whenever operator ids are not reused.
Checks some level of expression/schema consistency
Some sanity checks on SelectOperators
ql/src/java/org/apache/hadoop/hive/ql/hooks/OperatorHealthCheckerHook.java