Got infested by AdSense PSA?

Did you ever wonder, how often Google slips you public service ads instead of something that actually pays money? The problem with AdWords is, that advertisers can target countries and regions, meaning, that you might see real banners, while visitors from abroad might just get PSA. Of course, it's possible to use proxies to check, but thats neither reliable, nor convenient and most certainly not safe.
The solution to this problem is surprisingly trivial. Just create a placeholder image and upload it to your webserver. Next got to "Manage Ads" in your AdSense setup and select the unit, you suspect of misbehaving. Under "More Options" Select "Show non-Google ads from another URL" and enter the URL to your placeholder image. Save and wait for ten minutes. Now, every time a visitor would get PSA on that unit, your own webserver logs will inform you of the incident.