Chrome used to have this functionality, but they believe it's no longer necessary. The vast majority of rules can be quickly determined in a modern browser, meaning obsessing over rule efficiency is likely to cost you more in time & code cleanliness than it will buy you actual page speedups. For details, see the patch notes. I think Opera still has this, but I don't have browser on the computer I'm typing this from.
As the above case case shows though, you can't completely forget about selectors. However, this is a pretty simple case of a bad selector - ".ui-widget :active" implicitly uses the universal selector, which is still considered a pretty bad idea. The detached ":active" will match any element that is active in the entire DOM, then will check if it's within an element with the 'ui-widget' class. This case is covered in Google's efficient css guide, and more approachably by Chris Coyier.
In short: there aren't great tools for this, but fortunately you don't have to worry about it as much as you used to. Try to avoid the universal selector, getting needlessly specific, and favor classes and id's over element types.