what is the best way to find out actual box-shadow values?

I need to find the actual box-shadow element values of a css style rule that is written by the server. More specifically I need to know if the element has a visible shadow or not. which means, there may be a case where the shadow rule is not "none" but not displayed because the rgba value has 0 opacity, or the blur is 0 and the spread is negative.

any ideas?