No, and more or less anyone who tells you they can would be chalking one up for the Baring False Witness category. :wink
That would be like someone being able to spot a person with a hole in their heart. Since a Christian is a person who is completely committed to Christ and continually striving to be more like Christ, you would think they could be, however sometimes all the right actions may not be done in faith but rather in self promotion or pride. Just like finding a whole in a person's heart, knowing whether a person is a true Christian takes very careful examination of their life.
However, the effort to make sure we know who our brothers and sisters are can make for some great fellowship as the years go by.