I agree with everything that Irulan has said. Test ride bikes if you can. That's the best approach.

It's also helpful to list what your strengths and weaknesses are mountain biking, as well as the kind of terrain you will be riding most or like to ride the most to help narrow down your search. For example, if you like to rip downhill, you will likely want a more upright position (on flat ground) on a bike with a more slack head tube angle (lower number on the geometry charts), or you might prefer cross country riding and have to deal with a lot of tight switchbacks in which case you might want a bike with a steeper head tube and a lower front end.