finding the face of a popup

[Allan Gottlieb]

Some popups have become invisible; I believe they have
foreground=background=white.  Since I am using the color-theme
package, I do not claim this is an emacs bug and I am not asking for a
fix.

My question is how do I find the name of this face.  For an example
Start with emacs -Q
and execute M-x customize RET

Place mouse on ``Go to Group''

Popup ``Create customization buffer for this group'' appears with
perfectly fine black looking text on a light yellow background.

But how do I find the name of this face (the one with black on yellow;
not ``custom-link'', the face of ``Go to Group'').

thanks,
allan