Modern web applications are very dynamic in nature with rich user experience. Such applications typically use Web 2.0 and Asynchronous JavaScript and XML (AJAX) technologies. These applications are very different from conventional web applications as they use stateful C/S communication in an asynchronous fashion. The use agent is able to communicate with web server without explicit form submission. This capability makes the technology to support Rich Internet Applications (RIA). However testing such applications is a tedious task. This paper proposes a mechanism that automatically tests AJAX applications. The mechanism makes use of a crawler to capture the client side fields and generate a state-flow which is basis for the completion of automatic testing. A DOM tree is built based on the client side state which is useful to navigate to various parts of the tree in order to test the data. Different invariants of DOM tree are built for covering all states of the application. The proposed mechanism also makes use f plug-in concept for implementation of a tool that caters the present and future needs of the application. The practical results revealed that the proposed prototype is able to capture AJAX faults and report them to development team.