By default invisible characters (or whitespace characters) are displayed in Sublime Text 2 whenever you make a selection over some text, but occasionally it helps to see all invisible characters.
In order to do this in Sublime you need to open your preferences file (On OSx that is
⌘+,) and insert the line
By default it is set to
"selection" and the other option is for it to be
Why would you want to show invisible characters?
The reason I needed to turn on this feature was because I had copied and pasted a line of code from a website into a Ruby file and it had brought across with it some invisible characters, I was scratching my head for an hour trying to figure it before I posted a question to StackOverflow (http://stackoverflow.com/q/14863720/197302) and some kind soul pointed me in the right direction.