I have been testing some of the cheaper PPC w/ my site. What I do is submit an "entry" page that just provides a link to my real site telling the user to click to enter. It gives me an indication of who's clicking just to earn their .2 cents versus who is actually intrigued w/ my site. Also saves on bandwidth. My entry page is 1 K versus 40-50 K for my main page plus graphics.
And of course my "real" traffic that comes from search engines and what have you are of course entering the real page and never see my entry page.
If you have access to your log files you should be able to see where the traffic comes form. The use of a tracking URL will also make things easier to identify, and you won't have to create an 'entry page'. A tracking URL could look something like www.mypage.com?ref=overture .