How to tell if a browser is in "quirks" mode?
In Firefox and Opera you can determine if your browser is in "quirks mode" by checking page info.
Using document.compatMode, will tell you the mode you are in with most browsers.
In Chrome, Safari, and IE, run this javascript in the address bar:
javascript:window.alert('You are in ' + (document.compatMode==='CSS1Compat'?'Standards':'Quirks') + ' mode.')
(note that you'll need to re-type the javascript: portion after pasting into your address bar, due to recent security changes)
As you can query the render mode in JavaScript you can have a Bookmarklet which will tell you which render mode a page is using.
I found this render mode bookmarklet which works well for me:
javascript:m=(document.compatMode=='CSS1Compat')?'Standards':'Quirks';window.alert('You%20are%20in%20'%20+%20m%20+%20'%20mode.');
The full answer to your actual specific question of 'Is one single error enough to force it or do you have some leeway?' is that it totally depends on the error. For example,
<!-- Comment -->
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
will force quirks mode in IE 6 & 7 despite not really being an error (they just throw a total wobbly when the very first line of the file is not a declaration). A quick list of types/quirks can be found here
Try sticking the following line in your HTML for testing (very bad javascript behavious I'm passing on here - sorry...make sure this never goes live :)
<a href="javascript:alert(document.compatMode);">What mode am I?</a>
According to http://www.quirksmode.org/css/quirksmode.html : "The problem was that some pages written in quirks mode did have doctypes. Therefore each browser has its own list with doctypes that trigger quirks mode. See this browser comparison chart for an overview of these lists : http://hsivonen.iki.fi/doctype/"
Hope this helps
If you tell IE that it should be strict (via doctype) it will not change its mind halfway through the page.