Recently, Andrej blogged about whitespace usage in Sun’s JDK. He mentioned that the AnyEdit plug-in can be used to display the kind of whitespace (tabs vs. spaces) being used.
You might not be aware of the fact that Eclipse 3.3 has this feature as well. Here’s how to activate it: open Window –> Preferences –> General –> Editors –> Text Editors and activate Show whitespace characters. Voilà! Now you can see if your code contains tabs or whitespaces: