All of the above posts are great posts. I second those thoughts exactly and can add a few more.
I had a blast in med school on one particular rotation. It took me a while however to realize I just wasnt interested in the material. The actual knowledge base and disease profile just didnt interest me. In having to read about it, I realized I couldnt care less! This is highly unusual for me because I tend to be very interested in things and am not otherwise lazy in this manner. As said above, reading a few journals on a few specialties is an excellent suggestion. It will let you see exactly the type of mental debates and considerations that occupy these physicians. Does it seem interesting?
Another thing is not to let your romanticization of a specialty let you get carried away. Brain surgery for instance has that type of allure. Actually seeing the life of a neurosurgeon, and how most brain surgery aint brain surgery, is a sober wake up. You have to be able to accept the reality, which is very different from the romantic perception.
Its very difficult though discovering what its like to do X, Y, or Z. Unless you are actually doing it, you will never know what its like except as a short-term visitor. Inhabiting that world for longer though definitely helps, particularly the more active you are able to be. Like said above, seek out these opportunities. Try and determine what things that specialty will allow you to do, and how many of those things seem interesting to you. All specialties are limiting in one sense or another, so you have to see a day-to-day component of it that you will be happy doing for many years.