Global swatches are marked with a white triangle in the bottom right corner of the swatch. Book colors also have a black dot in this triangle.

These triangles are displayed everywhere swatches are used in UI: Appearance, Properties, Control... But for some reason they are omitted in tool panels, in Fill Stroke controls (and also Gradient).

I personally see no reason why. Sure users would benefit from knowing exactly if a color is global or not when they see the color chip anywhere.