Google began testing the Magnifying Glass Feature in the Diagnostic Tool in mid October, but it appears to now be live in all AdWords accounts now. Here is a screen shot on one of my sample campaigns, of the tool in action.
What does the tool do?
The Ads Diagnostic Tool can identify why a particular ad or group of ads may not be showing. In those cases where an ad is not shown, this tool provides recommended steps to help you get your ads up and running.
How do you activate it? Well, drill down to the keyword level within your Ad Group and then "Click the Keywords tab and point to the icon next to any keyword to see whether your ad is appearing."
More details at Google AdWords help.
The folks in the forums love the feature.
Forum discussion at WebmasterWorld.