C# webbrowser control - Confirm JavaScript function Executed and get response message

I have a web browser control, I want to open a webpage and execute a JavaScript function that used AJAX to send the data and if data meet some rules it send back a <div .... The <div will display a message whether submit succeed or not. I want to check if data sent and stored successfully then Invoke another JavaScript function.

But now I am using web browser DocumentCompleted event handler to find out page has been loaded and so on ...

I have to use that event handler.

What should I do to confirm JavaScript function executed and the response message is success?