When setting up Google AdWords campaigns, it can pretty difficult to guess which ad copy is going to perform the best. Sometimes even the tiniest, most unexpected changes can have a dramatic impact on ad performance. But how do you know what will be most effective?

A new Y Combinator-backed startup called MixRank wants to help take some of the guesswork out the process. The self-described "spy tool for contextual and display ads" offers businesses a glimpse at AdSense campaigns being run by other companies and shows which ones had the best performance.

MixRank indexes more than 93,000 sites running Google AdSense ads and figures out things like how many publishers ran the ad, when it was last seen, how frequently it's displayed and what it's average position in AdSense blocks is. It also breaks down top traffic sources for that site.

In the above example using ads run by Mint.com, you can see that the ad for "Personal Budget Worksheet" vastly outperformed the ad for "Family Budget Spreadsheet."

For small businesses especially, such a tool can be hugely valuable for better allocating those limited ad dollars and putting energy into the types of ad copy and traffic sources that will yield the highest ROI.