To see if a callback handler is working you must first create an item such as a screenshot or animated GIF, either through the API or Online Screenshot Tool. Then the callback handler test tool can be used to send this item to your callback handler. To use it simply go to Diagnostics
Click on one of the items you have already created in the Out column. Then click on the Send to Callback Handler link. Now enter the URL to your callback handler in the Callback URL field then fill in any of the optional fields such as a Custom ID, before pressing send. This will now send the selected item to your callback handler.