my first suggestion is to read about the new specialist; their bio, anything that they have published, etc. My second suggestion is to ask friends, co workers, neighbors, support group members who have had breast cancer for their experiences with area specialists. That has been extremely helpful for me. Finally, I would ask the current doctor about the specialist - how they know the doctor, what their working relationship is, how many patients they have sent on to that doctor, etc. If you are working with a nurse navigator, ask that person as well.
Best of luck - I was fortunate to be referred to an amazing doctor for a second opinion.