If you call your service in wcf test client, you will see < proto/> tag in the body of response
If you configure your client to use proto behaviour and call service with "inadvertently disabled protobuf", your answer will be null because of different serializers. You can check it in your test
var address = new EndpointAddress( url );
var binding = GetBinding( address.Uri );
var factory = new ChannelFactory<TService>( binding, address );
factory.Endpoint.EndpointBehaviors.Add( CreateProtobufEndpointBehavior() );
var client = factory.CreateChannel();
var answer = client.GetSomeInt();
AssertAnswerIsNotNull(answer);