There is no jQuery selector to do this. Javascript accesses the DOM, which doesn't directly "know" why an element has the computed style that it does. Further and because of this, DOM does not keep track of what the computed style would be without a given style declaration.
That said, one can disable a given style sheet:
http://jsfiddle.net/dPA9u/
You can use this method to turn off a given style sheet, check the computed style of your target element, then re-enable the stylesheet. You'll end up with the previous style, but the user won't see anything happen:
var ele = document.getElementById('my_div');
document.styleSheets[3].disabled = true;
var bg_color = window.getComputedStyle(ele).backgroundColor;
document.styleSheets[3].disabled = false;
alert('Original background color is: '+bg_color);
Try it: http://jsfiddle.net/GDSEK/1/
Putting it together, you can craft a script that loops through document.styleSheets
to find the target sheet, disable it, measure the computed style of the target element, then re-enable the target sheet.
This does not contend with inline styles -- but you can check those too by disabling all the style sheets, then checking on element.style
Documentation