There is of course the box spy but may not be what you want. In case you have are not acquainted with the box spy,Ctrl D, generates the Preview. on the second line of the Preview Panel, click on the spinner (arrow head) besides the spectacles icon and enable the box spy.  Passing the mouse over the processed page discloses the element, and if a class/id supplies its name.

However, if the Preview is being generated, the document source would be in the Editor would it not? In the source, right click on the opening tag of the element and select Related Styles and then select the appropriate. (an element may be styled in a linked style sheet and/or a style block at the top of the document). This will cause the linked style sheet to open in the Editor, and moves the cursor to that declaration as it will in the style block as appropriate.

Louise