Trailing whitespace is nasty, and there are various ways to go about catching it. There’s a neat way of running TextMate’s ‘strip trailing whitespace’ macro every time you save, for example, but that introduces an irritating lag if you’re twitchy with ⌘S like I am.

So instead, you can edit the language definitions for the languages you use:

{
  name = 'invalid.whitespace';
  match = '\s+$';
}

Then, open TextMate preferences, and under Fonts & Colours add a new colour named “Trailing Whitespace” and enter ‘invalid.whitespace’ as the scope selector. Make it red, or grey, or whatever you like, and stray whitespace will now be highlighted.