I shadowed a lot in 2nd and 3rd year of dental school.. at the specialty clinics.. Meet one or two residents that you get along with and they are happy to have you around. From there you meet attendings. I shadowed some attendings' practices. Started with perio clinic, went to endo, and private endo office, then ortho and private ortho office, then oral medicine office, and finally omfs. I went to all the specialty club meetings (before I had narrowed it down), and read on SDN and dentaltown. I asked residents for their opinions, and talked to some faculty... I regret not having known more about prosth... I only started meeting prosthodontists in my 4th year...
I found it hard to imagine what I could see myself doing. But there were surely procedures I was more interested in talking about with residents or my friends... Ultimately, interest in the procedures, patient relationships/patient demographics, and job prospects came into play --> OMFS. I decided late in my 3rd year. Made applications rather difficult, but worth it because I like feeling everything out before jumping in.