Use the reveal formatting pane and have it tell you the style source of formatting.
Shift F1. Click in text formatted in style A, then in text formatted in Style B.

Thanks Charles. I wasn't able to get that to work, but I did find a workaround in my particular case. If I change first style A and then style B to the same color, then thereafter updating style A's color cascades to B. Not sure how general this solution is (e.g., does it work on attributes other than color?).