There are a lot of reviews on the Den and varying experiences had... Search 'Fox Den' under the Vancouver Reviews section and you'll find more than enough to read. You're best to narrow it down by deciding which girls on the site you might be interested in and then searching under those names; you're likely to find something. As always, once you've decided who you think you want to see, try to stick to girls that've been well-reviewed already unless you're willing to gamble.
The bottom line is that you won't know unless you try it - good reviews on any one girl don't guarantee a good session every time, but it'll reduce your chances of having a bad one.