I normally only use spaces to indent but sometimes I have to edit a file that contains tabs and I don’t want to break the existing indentation. In this situation Emacs’ whitespace-mode is very useful for showing exactly which whitespace characters are present in a file. But I don’t want to enable it all the time so I use this little function to check automatically if a file contains tabs, and if so enable whitespace-mode and also indent-tabs-mode so that any new lines are indented with tabs as well.

(defun check-for-tabs ()
  "Enable whitespace-mode and indent-tabs-mode if the buffer
contains tabs."
  (when (save-excursion
          (goto-char (point-min))
          (search-forward "\t" nil t))
    (whitespace-mode +1)
    (setq-local indent-tabs-mode t)))
(add-hook 'find-file-hook #'check-for-tabs)