Q:

In Azure IoT hub - where do i see MQTT messages that has been sent from a device?

im trying to send messages over the MQTT protocol to Azure IOT hub. 
Im using this code in NodeJS:
var clientFromConnectionString = require('azure-iot-device- 
mqtt').clientFromConnectionString;
var Message = require('azure-iot-device').Message;

var connectionString = 'HostName=myhostname.azure- 
devices.net;DeviceId=Arsenal;SharedAccessKey=mysharedaccesskey';

var client = clientFromConnectionString(connectionString);

var connectCallback = function (err) {
  if (err) {
    console.error('Could not connect: ' + err);
  } else {
    console.log('Client connected');
    var message = new Message('some data from my device');
    client.sendEvent(message, function (err) {
      console.log("Message sent!");
      if (err) console.log(err.toString());
 });

client.on('message', function (msg) {
  console.log(msg);
  client.complete(msg, function () {
    console.log('completed');
   });
  });
 }
};

client.open(connectCallback);

This prints "Client connected!" and "Message sent!"
Maybe this is a really silly question, but where in IoT Hub can i see the message that has been sent?

A:

Three suggestions:
Azure CLI

At a PowerShell prompt type

az login

The az login command will open a new web browser window and ask your to log into your Azure Subscription account.  
Next we need to generate a SAS token.  We can do this by running this command in the Azure CLI window:

az iot hub generate-sas-token --duration  -n 

Now we have our SAS Token, we can start monitoring the hub messages by running the following command:

az iot hub monitor-events --hub-name  

for more details please read: https://github.com/AzureIoTGBB/How-To-Monitor-Azure-IoT-Hub-with-Azure-CLI-2.0
Azure Function

Create a Function App by clicking the below in the Azure Portal “Create New” blade. This is the “container” which holds your functions.
Once the Function is created, navigate to the “Create new function from template page”:
Select IoT Hub (Event Hub) and Javascript or C# as the language
Conveniently, Azure Functions will create the connection with IoT Hub for you after selecting this template,
Next you just have to click on the "new" button and select the proper IoT Hub instance:
Click "Create" and the Function just created will be triggered by the IoT Hub events
The function is created with boilerplate code     console logging the messages

Device Explorer for IoT Hub Devices
On the read me of the repo it describes how to download, build and use the Device Explorer tool:

You can either download a pre-built version of Device Explorer or build it yourself.
Configure the IoT Hub Connection 
Click on the Management tab 

Also there is a another thread that can be useful since it describes how to use it:  How to test Messages arriving in Azure IoTHub

