Hi Joe,
I'm assuming you are asking about IE8? Based on
http://stackoverflow.com/questions/1328963/detect-ie8-compatibility-mode there are two ways.
1) IE8 includes a trident token in the User-Agent string regardless of compatibility mode.
See MSDN for more details:
http://blogs.msdn.com/ie/archive/2009/01/09/the-internet-explorer-8-user-agent-string-updated-editio...
IE7 on Windows Vista
Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.0)
IE8 on Windows Vista (Compatibility View)
Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.0; Trident/4.0)
IE8 on Windows Vista
Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 6.0; Trident/4.0)
2) You can also check the value of document.documentMode which will have a value of 8 if true IE8 mode and 7 in IE7 mode. The user agent string will be identical in both cases and will still state IE8. Note that this JavaScript variable is only available in IE8.
Hopefully that is what you are looking for. I'm guessing you are wanting to add a load rule that suppresses a tag based on compatibility mode. If this is the case you'll need to add some logic to a JavaScript extension scoped to Pre-Loader that outputs a global JS variable and then reference that variable in the load rules.
Please let me know if this helps.