I'll usually turn to whomever I'm walking with, slightly nod toward the Spanish-(or any other language, for that matter)-speakers, rattle off a little bit of Anglo-Saxon or Old Icelandic, and then grin knowingly. It's soooo funny to watch them get furious over what they themselves were just doing.
I remember one time a few years ago where I was working these girls were talking in spanish and a newcomer as white as white can get said something in spanish to them indicating she knew what they were saying and shut up.