There is great debate about when a web application is "thick" versus when it is "thin". As a result, application thickness has a spectrum rather than a boundary. However, here is a rule of thumb to help make the determination -- the "back button test". If the back button doesn't work in a web app, it is almost certainly a thick app. If it does work, then the app is quite likely to be thin. The test isn't perfect, but it works quite well in practice.