you find out if its available when you try to add it to your cart. everything that you add to your cart will remain there until checkout unless there is 15 minutes of inactivity. at that time, everything will be removed from the cart and returned to inventory so others can order it.
i think they do track their inventory instantaneously. when i first logged onto the site, the chain vetter skirt was still available, and by the time i got done shopping 30 minutes later, it was listed as sold out.