I was working on some demos here at Corillian and I needed to check the size of the ViewState on a page. There's lots of ways and lots of tools that can help you do this. Certainly FireFox's Web Developer Toolbar, while not specific to ViewState, can help you analyze a page.
Nikhil Kothari's Web Development Helper is also very good at this, but requires a little more setup and ASP.NET 2.0. An awesome tool, though.
I stumbled on Binary Fortress Software's ASP.NET ViewState Helper (their support forum is brand new, but if you have trouble go there).
What a slick tool! A 300k download, and works on ASP.NET from the outside - by futzing with IE's object model. It watches you surf and collects statistics. You can then double click on any of the pages and view the decoded ViewState.
I like a tool that does its job, doesn't run inside of IE (I've had it with Browser Helper Objects), and gets out of the way when I'm done. Recommended. This tool is a very useful addition to your c:\utils.
(BTW, their Web Performance Monitor has promise too, but I haven't looked at it yet).
Scott Hanselman is a former professor, former Chief Architect in finance, now speaker, consultant, father, diabetic, and Microsoft employee. I am a failed stand-up comic, a cornrower, and a book author.
Disclaimer: The opinions expressed herein are my own personal opinions and do not represent my employer's view in any way.