There are two different test modes targeting different use cases.
The main one people use is the Status -> Display Testing, but this is more targeting complete display. There are ways to select just channel ranges for various models/strings, but the information provided by that is "limitted".
The other test mode would be on the Channel Outputs configuration page. For panels, there is a "Test Pattern" button that will display a specific pattern on the panels so you can tell if things are configured correctly, panel type is correct, etc... For string based capes, there are various options for putting string # markers, pixel count markers, etc...