How to test the `Mosquitto` server?

If you are using Windows, open up a command prompt and type 'netstat -an'.

If your server is running, you should be able to see the port 1883.

If you cannot go to Task Manager > Services and start/restart the Mosquitto server from there. If you cannot find it here too, your installation of Mosquitto has not been successful.

A more detailed tutorial for setting up Mosquitto with Windows / is linked here.

To test and see if you can access your MQTT server from outside world (outside of your VM or local machine), you can install one of the MQTT publishing and monitoring tools such as MQTT-Spy on your outside-world machine and then subscribe for '#" (meaning all the topics).

You can follow this by the method @hardillb mentioned in his answer above and test back and forth such as this:

On the machine with Mosquitto Server running,

On the outside-word machine with mqtt-spy running,

I have mainly mentioned mqtt-spy since it's multi-platform and easy to use. You can go with any other tool really. And also to my knowledge to run the mosquitto_sub and mosquitto_pub you need to have mosquitto-clients installed on your Linux machine running the test (in my case Ubuntu) which can be done easily by,

sudo apt-get install mosquitto-clients

Start the Mosquitto Broker
Open the terminal and type

mosquitto_sub -h 127.0.0.1 -t topic

Open another terminal and type
mosquitto_pub -h 127.0.0.1 -t topic -m "Hello"

Now you can switch to the previous terminal and there you can able to see the "Hello" Message.One terminal acts as publisher and another one subscriber.

In separate terminal windows do the following:

1. Start the broker:

   mosquitto
   

2. Start the command line subscriber:

   mosquitto_sub -v -t 'test/topic'
   

3. Publish test message with the command line publisher:

   mosquitto_pub -t 'test/topic' -m 'helloWorld'
   

As well as seeing both the subscriber and publisher connection messages in the broker terminal the following should be printed in the subscriber terminal:

test/topic helloWorld

EDIT:

It is worth pointing out that from v2.0.0 of Mosquitto it will only be listening for connections on the loopback interface by default. If you want to access the broker from machines other than the one it is installed on you will need to edit the config file (and pass it to the broker with the -c option e.g. mosquitto -c /path/to/mosquitto.conf) to enable listening on other interfaces. Details can be found in the v2.0.0 release notes here