How to decode viewstate

Use Fiddler and grab the view state in the response and paste it into the bottom left text box then decode.

Here's an online ViewState decoder:

http://ignatu.co.uk/ViewStateDecoder.aspx

Edit: Unfortunatey, the above link is dead - here's another ViewState decoder (from the comments):

http://viewstatedecoder.azurewebsites.net/