In many cases, it makes sense to test a payload before sending it to a web service.
Assuming that you already have a source-to-destination flow which sends a payload to the HTTP endpoint, to test the payload:
Step 1. Replace the destination connection with a connection to server storage, while keeping the same destination format.
Step 2. Execute the flow manually and check the output.
Step 3. Once you are satisfied with the output, swap the destination connection with the connection to the web service.
Alternatively (actually preferred but not always possible), you can simply test the transformation without running the flow.