It depends on how people get jobs. For vascular surgery, the vast majority are through word of mouth. The field is small enough and the total number of positions is small enough that you learn about job openings when talking to people in your field or having your bosses ask around for you. To this end, a lot of recruiting, especially for junior faculty/fresh graduates happens at conferences or local phone calls. Only a very small minority of positions are advertised, unless you are out in the boonies. Thus, to find out about the job market, you have to ask people ahead of you or already in the field and looking around. I keep loose tabs on all of the fellows who graduate. I ask them how their job searches are going, what kind of offers they are getting etc. Something else that matters is whether you are talking academics or private practice. There are a very limited number of academic positions available in each city, private practice not so much.
In short, it is extremely difficult for a pre-med or even medical student to gauge these this, other than the big global trends. A large part of where you practice will be where you do your residency. If you 'need' to be in Boston, you shouldn't be aiming for competitive specialties, regardless of what the job market looks like post-residency.