In Zendesk Chat, how can I simulate a chat for testing?
From the Chat dashboard, select Visitors > Simulate Visitor. A new window opens from which you can interact with your Chat widget as a visitor.
The Simulate Visitor button will not show up if there are any active visitors at that time. To get around this, initiate a test chat acting as an end user from the page your Chat widget is hosted on, or simply wait until there are no active visitors to see the Simulate Visitor button again.
For more information, see the article: Best Practices: How to test a Chat trigger.