Quickly know when you're in quirk mode

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:

javascript: window.alert(document.compatMode);

CSS1Compat means the browser is not in quirk mode.

Simple and efficient, but of course not with frames (which you shouldn't use anyway!)