(setq mew-ask-charset '("us-ascii" "iso-2022-jp"))

などと設定して、これ以外の文字を含むメールを送信しようとすると、
“utf-8 is used. OK?” と聞かれるが、本文のどこに該当の文字が含まれて
いるのか経験と勘(笑)で調べるしかなかった。そこで、“n” でキャンセル
した後、該当の文字の背景を紫色にするようにしてみた。

;; 「UNICODE 文字」の face を定義
(defvar mew-unicode-char-face-list
  '((((class color) (type tty)) (:bold t))
    (((class color) (background light)) (:background "purple"))
    (((class color) (background dark))  (:background "purple"))
    (t (:bold t))))
(defface mew-unicode-char-face mew-unicode-char-face-list
  "UNICODE 文字"
  :group 'mew-highlight)
(defvar mew-unicode-char-face 'mew-unicode-char-face)
(face-spec-set 'mew-unicode-char-face mew-unicode-char-face-list)

(defadvice mew-encode-undo
  (after
   mew-encode-undo-after-advice
   activate)
  "「UNICODE 文字」を強調表示"
  (if mew-ask-charset
      (let* ((beg (progn (mew-header-goto-body) (point)))
             (end (or (mew-attach-begin) (point-max)))
             (unicode-char-regexp (mew-cur-unicode-char-regexp beg end)))
        (if (stringp unicode-char-regexp)
            (save-excursion
              (mew-elet
               (goto-char beg)
               (while (and (<= (point) end) (re-search-forward unicode-char-regexp end t))
                 (put-text-property
                  (match-beginning 0) (match-end 0) 'face 'mew-unicode-char-face))))))))

(defun mew-cur-unicode-char-regexp (beg end)
  (let (pos unicode-char unicode-char-list)
    (save-excursion
      (goto-char beg)
      (while (< (setq pos (point)) end)
        (if (and (mew-unicode-char-p pos)
                 (setq unicode-char (char-to-string (char-after pos)))
                 (not (member unicode-char unicode-char-list)))
            (setq unicode-char-list (cons unicode-char unicode-char-list)))
        (forward-char 1)))
    (if (consp unicode-char-list)
        (concat "[" (mapconcat 'identity unicode-char-list "") "]+"))))

(defun mew-unicode-char-p (pos)
  (let* ((char (char-after pos))
         (eight-bit-p (and (not enable-multibyte-characters) (>= char 128)))
         (charset (if eight-bit-p 'eight-bit
                    (char-charset char))))
    (and (not (eq charset 'ascii))
         (not (eq charset 'japanese-jisx0208)))))

さらに draft-mode で C-c C-l した時も色付けさせる設定。

(defadvice mew-highlight-body-region
  (after
   mew-highlight-body-region-after-advice
   activate)
  "「UNICODE 文字」を強調表示"
  (if (and mew-ask-charset
           (eq major-mode 'mew-draft-mode))
      (let* ((BEG (ad-get-arg 0))
             (END (ad-get-arg 1))
             (unicode-char-regexp (mew-cur-unicode-char-regexp BEG END)))
        (if (stringp unicode-char-regexp)
            (save-excursion
              (mew-elet
               (goto-char BEG)
               (while (and (<= (point) END) (re-search-forward unicode-char-regexp END t))
                 (put-text-property
                  (match-beginning 0) (match-end 0) 'face 'mew-unicode-char-face))))))))

追記(2009-03-02):
mew-cur-unicode-char-regexp() と mew-unicode-char-p() を変更しました。
前者は検索回数の削減、後者は Unicode 判断の変更です。