Well, it seemed that the problem was that the face properties which I wanted anzu-mode-line to take were still unspecified at the time of setting it, so I kept on messing around with hooks and with-eval-after-load until I came up with this
Your question isn't clear to me. But I guess you're asking how to tell what faces actually have a visual effect on a given character.
Not sure this answers the question at all, but:
Node Displaying Faces of the Elisp manual says how various faces affect the display of a character they act on.
C-u C-x = tells you what faces are used on the character after ...
Set variable hi-lock-face-defaults to the names of the faces you want.
(add-to-list 'hi-lock-face-defaults "highlight") ; Add a face (`highlight`).
(setq hi-lock-face-defaults ; Set to just these 3 faces.
'("dired-flagged" "highlight" "hi-yellow"))
(setq hi-lock-face-defaults ; Add multiple faces.