What can you see if a user opts out of Google Analytics tracking using the browser plugin, can you see that they came, but not much else ie. they show up as a direct visitor, or do they not show up at all (although you can always see them in the server logs.)
That data will not be reported to you (they do not show up at all).
In fact you can set this up programmatically as well - Tracking Basics. When you do execute the disable tracking command or if the visitor has a disabled Javascript or disabled tracking, then GA will not report that visit.
