Well, that is a whole topic n itself. In my case, I had only been told what the differance in a cf frame was, compared to aluminum and steel. So, I went to the LBS and test rode a couple differant bikes with cf frames. My purchase of my frame was basically just based on the feel of other similar frames, armed with the knowledge of kowing exactly the geometry and measurements I was looking for. The only unknown factor in my case, was the feel of the ride when built, since cf frames tend to differ a bit between differant brands and designs.
After my build was complete, my expected performance, and feel of the ride was exactly as I had anticipated. So, I was lucky in that respect. The key, is to do lots of research, and not be afraid to ask lots of questions from differant people to get a whole wealth of opinions and knowledge. Then, you just form your own personal opinion, based on what you personally want, and go for it.