What is XSS?

Cross Site Scripting (XSS) is an type of a attack when the attacker inserts browser executable code within a HTTP response. Now the injected attack is not stored in the web application, it will only affect the users who open the maliciously crafted link or third-party web page. A successful attack allows the attacker to access any cookies, session tokens, or other sensitive information retained by the browser and used with that site

You can test by detecting user-defined variables and how to input them. This includes hidden or non-obvious inputs such as HTTP parameters, POST data, hidden form field values, and predefined radio or selection values. You then analyze each found vector to see if their are potential vulnerabilities, then when found you craft input data with each input vector. Then you test the crafted input and see if it works.