If you want to be able to spot the difference between a tab
and a space character you could highlight tabs like this.
(defface tab-face '((t (:background "red")))
"highlight tabs")
(font-lock-add-keywords 'tacl-mode '(("\t" 0 tab-face prepend)))
;; ^^^ substitute your mode name here
;; spaces can be highlighted too if you really want.
(defface space-face '((t (:background "grey" )))
"highlight whitespace")
(font-lock-add-keywords 'tacl-mode '((" " 0 space-face prepend)))
;; ^^^ substitute your mode name here
If you only want to see trailing whitespace I use this
(defun stw () "Toggles `show-trailing-whitespace' on/off"
(interactive)
(setq show-trailing-whitespace (not show-trailing-whitespace)))