To help you separate real callbacks from test callbacks, as well as help you possibly make your system a bit more secure, the callback tester sends an extra $_GET parameter to the callback URL.
All callbacks from this tester include testmode=true. For example, we would send a callback to: https://www.yourdomain.com/voteprocessing?callback=CONTENTHERE&ip=VOTERIP&testmode=true
Do you need help setting up the callback? Take a look at the documentation.