Intercept onclick handlers and wrap them with your own intercept code.

onclickintercept.js

//Assume this was written before you got here.
document.body.onclick = function(){ alert(this) };

// Here's what you'd do.
function wrapper(elem, fn) { 
  return function() {
    //You can put your intercept code here.
    console.log('intercepted');
    
    //Then call the original function with the usual arguments it would expect.
    return fn.apply(elem, arguments);
  }
}

if (!!document.body.onclick) {
  document.body.onclick = wrapper(document.body, document.body.onclick);
}
//Now when you click on the body your console.log will run before the original onclick handler.