Quoting from a guy named "Mong" teached you how to check the ad click in adsense. Here is the modified version of javascript
var elements;
if(document.getElementsByTagName) {
elements = document.body.getElementsByTagName("IFRAME");
} else if (document.body.all) {
elements = document.body.all.tags("IFRAME");
} else {
elements = Array();
}
for(var i = 0; i < elements.length; i++) {
if(elements[i].src.indexOf('googlesyndication.com') > -1) {
elements[i].onfocus = adclick;
}
}