I have configured the IDE to indent via tabs (size=2).  Sometimes while editing files (especially in a shared development environment) I'll notice that someone used two spaces instead of a tab, but it's not readily apparent.  

One solution is to show tabs visually, perhaps as a very light grey arrow.  Or to be more clever, the IDE could only make tabs or spaces visible when they aren't expected, e.g. spaces indenting lines in a normally tab-indented file could show up as yellow, or similarly highlight tabs indenting a normally space-indented file.  

Also, maybe visually draw tabs whenever they appear in the middle or end of a line (not as an indent).  I don't think there is any reason for them to (ever?) be used there.