How are you tracking coverage?

I am defining coverage as the percentage (%) of the time an ad appearsin search engine, e-zine, or other media.

I am not referring to click-through rate or conversion because those can be monitor with tools like AdWords Campaign Summary. It is easy to say that "we have one million readers" but all one million readers might not be online when my ad(s) are display.

So, my question is "how are you tracking coverage(reach)?" How do you know how many people actually seen your ad(s)?