How do I know that a configuration is set up if I can't see it on my web.config file?

Question

Hello,

I'm walking through the Series on "How to use charts with ASP.NET" (Scott Michell). Somewhere, he says:

...Both of the above configuration additions are automatically added to Web.config when you add a Chart control to a web page for the first time in your project...

I've checked on my web.config, but I didn't see anything. So, I manually added them. Now, when I try to run the program, I get the following error:

...The server tag 'asp:Chart' is ambiguous. Please modify the associated registration that is causing ambiguity and pick a new tag prefix...

Line 4:  <%@ Register Assembly="System.Web.DataVisualization, Version=4.0.0.0,
         Culture=neutral, PublicKeyToken=31bf3856ad364e35"...

It looks like things are working the way the author said. But, How do I know that a configuration is set up if I can't see it on my web.config file? Do I need to run first into an error?

Thanks for helping.