The most basic and straightforward way is to use Google Analytics. And I'm not just talking about installing Google Analytics. I'm talking about using the UTM (short for Urchin Tracking Module, which relates to the old Urchin analytics system that Google acquired to create its tracking tool) variables that exist in Google's Analytics to ensure that you know where they're coming from.
That’s true Thomas – this can happen when going after very competitive keywords. To avoid that you can just grab the first subpage you see ranking – subpages most of the time won’t have a lot of brand searches associated with them/you’ll see true topic value. It may be lower than normal, but in general can’t hurt to have a passive calculation when making arguments of what you might achieve.