Depends upon how you are defining things, when it comes to technical skills it is pretty easy but when it comes to the "good fit" side of things, not so much.
In regards to technical skills, if you construct the interview(s) correctly then you should know if they are talented or bright enough that they are worth training up to speed. If you are doing a mixture of white board coding problems, design problems, and behavioral questions you should get a fairly good idea of the candidate's skills. Likewise, having a minor goof on a white board problem is not such a bit deal if they are able to work though it and if they make a mistake it might be a good thing as it can give you a chance to see how they react to feedback and work through problems.
In regards to the "good fit" side of things, quite simply it depends upon what you are defining as a good fit in the group. If you get the group's expectations out up front to the candidate then if they think they aren't a good fit they may filter themselves out of the position for you (some what dependent upon the economy though). But that said, it may take a long time to tell if someone is a good fit for the group as people tend to take some time to relax at the office and many behave differently at the office than they do at home.