Forum Moderators: open
something like
www.mysite.com?source=ovkey1
or
www.mysite.com?source=gadword1
works pretty well, sometimes I also target specific terms to specific pages and that allows me to track clicks/purchases to that single page.
Regardless it is never 100%.
Some just track how many hits and conversions per either KW or engine, while others track each KW cost per engine and how much that KW made in end conversions giving you very detailed KW per engine ROI.
It depends if you want to install it yourself, just install a code on your page and let someone else crunch the numbers, and the level of detail you want to track.
If you are already using a site stat program, there's probably a way as suggested by jatar_k of adding a?widget+adword to the url depending on what program your using.