How do you detect when a network runs out of banner ads to show?

[Raymond]

I am new at CPM ads, so please be gentle.

I was told that if I have a lot of impressions I might get default banners that don't pay me anything like adsense's PSA, and I should sign up multiple CPM networks so that when one runs out of ads it can switch to another so I will always get paid for the impressions that I do.

Is there a script that can do this or do I have to manually change the ad network myself when I see the default ads.

[ronin]

Many CPM networks allow you to specify a redirect as an alternative to showing a default.

In some cases you specify the redirect through the interface (ValueClick Media is one example) or through the code that goes on your website (Google AdSense is an example).