Your script that will handle the IPN post data will be called whether Test Mode is ON or OFF.
Make sure that you have entered the correct address to your script in the Alert URL field in your account in the IPN setup.
If you do not receive your IPN post data, here are some test that you can do to check that you are receiving the IPN properly.
1. Make sure that your AlertURL is not an alias, it has to be an actual URL.
2. Please check if your server/host provider is blocking our public IP
3. Please check the firewall settings on your server that might be blocking our IPN post.