[ale] Make emacs show "invisible" characters

[JK]

Hi folks,

I used to know how to do this, but neither my brain
nor Google is my friend today.

I'm editing a source file that has line-ending issues --
it's got a mix of DOS and Unix line endings.  I know
there is a way to provoke emacs to explicitly display
the line-ending characters (^M, ^J, etc) and other
normally "invisible" whitespace, but I can't find it.
Any clues?

Thanks,

-- JK