Where is whitespace visibility toggled in VisualStudio 2010?

Edit | Advanced | View white space (Ctrl-R, Ctrl-W)

If you're using the Express version of Visual Studio, it's possible that you need to enable "Expert Mode" to see the menu option (or it might not be there at all for all I know - I don't have Express installed to check).

A related setting is in Tools / Options / Text Editor / All Languages / Tabs. If everyone agrees on a common setting, it simplies readability between users.