There are 4 regions where text may be displayed in Emacs, here is what you can do to inspect each of them.

• The mode-line: See the value of mode-line-format.
• The header-line: See the value of header-line-format.
• Buffer: If the face is in a region of the buffer you can't reach, see the value of (buffer-string).
• Minibuffer: If point is currently in the minibuffer, you can do M-: (buffer-string), and that will describe the contents of the minibuffer. You may need to (setq enable-recursive-minibuffers to t) first.

These methods will give you the entire contents of these regions, so it may take some reading to actually find the face you're looking for, but it will certainly be there. Most importantly, it will describe even regions you cannot reach.
For instance, let's say I want to know which face is used in the find-file prompt.

1. Invoke find-file with C-x C-f.
2. Describe the minibuffer with M-: (buffer-string)

That will give something like the following output, and right there at the end is the face I was looking for.

#("Find file: ~/" 0 11 (front-sticky t rear-nonsticky t field t read-only t point-entered minibuffer-avoid-prompt face minibuffer-prompt))

There are 4 regions where text may be displayed in Emacs, here is what you can do to inspect each of them.

• The mode-line: See the value of mode-line-format.
• The header-line: See the value of header-line-format.
• Buffer: If the face is in a region of the buffer you can't reach, see the value of (buffer-string).
• Minibuffer: If point is currently in the minibuffer, you can do M-: (buffer-string), and that will describe the contents of the minibuffer. You may need to (setq enable-recursive-minibuffers to t) first.
• Fringe or Margin: None that I know of.

These methods will give you the entire contents of these regions, so it may take some reading to actually find the face you're looking for, but it will certainly be there. Most importantly, it will describe even regions you cannot reach.
For instance, let's say I want to know which face is used in the find-file prompt.

1. Invoke find-file with C-x C-f.
2. Describe the minibuffer with M-: (buffer-string)

That will give something like the following output, and right there at the end is the face I was looking for.

#("Find file: ~/" 0 11 (front-sticky t rear-nonsticky t field t read-only t point-entered minibuffer-avoid-prompt face minibuffer-prompt))

There are 4 regions where text may be displayed in Emacs, here is what you can do to inspect each of them.

• The mode-line: See the value of mode-line-format.
• The header-line: See the value of header-line-format.
• Buffer: If the face is in a region of the buffer you can't reach, see the value of (buffer-string).
• Minibuffer: If point is currently in the minibuffer, you can do M-: (buffer-string), and that will describe the contents of the minibuffer.

These methods will give you the entire contents of these regions, so it may take some reading to actually find the face you're looking for, but it will certainly be there. Most importantly, it will describe even regions you cannot reach.
For instance, let's say I want to know which face is used in the find-file prompt.

1. Invoke find-file with C-x C-f.
2. Describe the minibuffer with M-: (buffer-string)

That will give something like the following output, and right there at the end is the face I was looking for.

#("Find file: ~/" 0 11 (front-sticky t rear-nonsticky t field t read-only t point-entered minibuffer-avoid-prompt face minibuffer-prompt))

There are 4 regions where text may be displayed in Emacs, here is what you can do to inspect each of them.

• The mode-line: See the value of mode-line-format.
• The header-line: See the value of header-line-format.
• Buffer: If the face is in a region of the buffer you can't reach, see the value of (buffer-string).
• Minibuffer: If point is currently in the minibuffer, you can do M-: (buffer-string), and that will describe the contents of the minibuffer. You may need to (setq enable-recursive-minibuffers to t) first.

These methods will give you the entire contents of these regions, so it may take some reading to actually find the face you're looking for, but it will certainly be there. Most importantly, it will describe even regions you cannot reach.
For instance, let's say I want to know which face is used in the find-file prompt.

1. Invoke find-file with C-x C-f.
2. Describe the minibuffer with M-: (buffer-string)

That will give something like the following output, and right there at the end is the face I was looking for.

#("Find file: ~/" 0 11 (front-sticky t rear-nonsticky t field t read-only t point-entered minibuffer-avoid-prompt face minibuffer-prompt))