While debugging an application rendering issue yesterday (yes, I'm back to a bit of asp.net work, with CSS adapters and asp.net AJAX, oh the joy!) I came up with a very quick way to know when you're in quirk mode.
In the address bar, type the following:
CSS1Compat means the browser is not in quirk mode.
Simple and efficient, but of course not with frames (which you shouldn't use anyway!)