Non-printing characters - [edit] Whitespace

Can we have a command to see non-printing text/characters? I am specifically thinking of spaces. At the beginning of lines a stray space can have considerable effects on the rendering of text. It can be quite difficult and/or tiresome tracking down where the errant space is that is causing LI/UL/Checkbox/code errors.

[edit] @deathau posted a request for visible whitespace - this is a much better waay of asking for the same thing I was on about…