How can I determine Adsense's visibility

As we know that we have to create a new profile field to start Adsense. When it comes to the Revenue sharing, I hope Adsense can determine to show or not to show the Ads. In the other words, if the user have input his client ID, the ads will be shown on his node, Otherwise No ad will be shown on his node.
I think this can be done with a simple php code in block settings. Who can tell me how to?
Thank you a lot.