How to debug a WebMethod in asp.net form application

I have gone though  article 

http://www.aspsnippets.com/Articles/Send-and-Receive-JSON-objects-to-Web-Service-Methods-using-jQuery-AJAX-in-ASPNet.aspx

Sending and receiving json data.

There i have noticed you are debuging webmethod to get the properties of city object.

In my application i am making a ajax call to a webmethod and getting a error,  in firebug it shows internal server error . I believe there must be some bug in my webmethod. But i dont know how to debug it setting break point in my code didn't help as it overlooks the break point.

Kindly Help me!!