This is very simple to figure out.
Step One: Log into AdMob
Step Two: Click "Analyze" at the top.
Step Three: Compare your total ad impressions for any given period on the "Monetize" tab to the total screen views on the "Analyze" tab (for the screens which display ads).
For example, for yesterday on one of my apps, I had 29,332 ad requests via the Monetize tab (not including the interstitial).
Over on the Analyze tab for that same app, I had 41,882 total screen views on that app.
Divide that up and you've got 70.03%
So you can check this on your own, I believe, unless there's flaw in my logic here. But it always comes out to about 70%.