How to differentiate errors using GraphQLClient in Flutter

Question

I have a graphql service layer for my app as follows

class GraphQLService {
  GraphQLClient client;

  GraphQLService({String jwt}) {
    createClient(jwt);
  }

  Future<QueryResult> performQuery(String query,
      {Map<String, dynamic> variables}) async {
    QueryOptions options = QueryOptions(
        documentNode: gql(query),
        variables: variables,
        errorPolicy: ErrorPolicy.all);
    QueryResult result = await client.query(options);
    if (result.exception != null){

    }
    return result;
  }

  Future<QueryResult> performMutation(String query,
      {Map<String, dynamic> variables}) async {
    MutationOptions options = MutationOptions(
        documentNode: gql(query),
        variables: variables,
        update: (Cache cache, QueryResult result) {
          if (result.hasException) {
            print(['optimistic', result.exception.toString()]);
          }
        });
    QueryResult result = await client.mutate(options);
    if (result.exception != null){
      // refresh token here... what if no internet connection ?
    }
    return result;
  }
}

However after I leave my app idle for about an hour I notice the jwt token expired, so I am thinking to add checking to see if the error is indeed due to the expired token. But I cant seem to differentiate the cause of error since result.exception does not give any error code whatsoever.

Any better way to go about this ?