There are a number of ways to test a HiveMQ Cloud environment, ranging from the HiveMQ CLI Tool, to the HiveMQ WebSocket client, to the numerous clients libraries that allow the custom development of an MQTT client to fit your particular needs.
In the case of wanting to test the functionality of a new Cloud deployment, there is a baked-in Web Client functionality that will let you subscribe to topics and publish messages with very little initial setup. Simply navigate to your HiveMQ Cloud deployment, select the cluster you would like to manage, and then select the ‘web client’ option from the top of the page. Click ‘Connect with generated credentials’ to automatically create credentials for the test session, and use the publish and subscribe features on this page to test messaging.
If you happen to be new to MQTT as well, be sure to check out the MQTT essentials series for some additional insight into MQTT as a whole.