In some existing code there is a test to see if the user is running IE, by checking if the object Browser.Engine.trident is defined and returns true.
But how can I deter
This is probably going to get voted down, because it's not directly answering the question, but... You should not be writing browser-specific code. There's very little you can't do while coding for most widely-accepted browsers.
EDIT: The only time I found it useful to have conditional comments was when I needed to include ie6.css or ie7.css.