From Ross-Tech Wiki

4.1 Why and How should I test VAG-COM prior to Activating it?
All of our Current Interfaces that have built-in Activation, are fully Registered and Activated once you test them on a car. With these Interfaces, the license is built in, you do not have to Activate VAG-COM, but obviously you'll want to test VAG-COM when you get it to make sure everything works properly.
For our older Interfaces without built-in Activation, or for 3rd party Interfaces:
As per our Guarantee no refunds will be issued once you have received an Activation Code from us. Therefore, it is in your best interest to test VAG-COM with your PC and your vehicle prior to requesting an Activation. First, Download and install VAG-COM on your PC. Find your vehicle on the Applications Page to see which control modules are in your car. Connect your Interface to your car's Diagnostic Port and to your PC. Turn the ignition ON (turn the key so the Instrument Cluster lights up, but do not start the car -OR- start the car). Go into the Options screen, select the correct port, and click the TEST button. Ensure that VAG-COM finds your interface, you should see a message that says Interface Found. Now click SAVE. Select one of your car's Control Modules (note: your car will not have all the Control Modules listed in VAG-COM, only the ones on your Applications Page). After VAG-COM has Connected, check Fault Codes. Repeat the process for each of your car's Control Modules. If you encounter problems, please try These Suggestions first. If you still have problems, feel free to Contact Us with full details about your PC and vehicle, preferably while you are in front of both.