Its hard to say, and can be a problem. Ask your LBS if they offer any type of returns policy. My LBS will allow you to return a bike for any reason within two weeks after the sale (if its in like-new condition). That should give you enough time to know if the bike fits you, and is what your looking for.
In all honesty, i dont think you will know the bike is perfect for you for atleast 6 months. Even more so if its your first mtn bike.
BTW, for $1500+ I would demand a full XT groupo.