Select the DNA Match so that it's is on its own profile page where you can see the tree.

There are three tabs above are Tree, Ethnicity and Shared Matches.

Select Shared Matches and all the matches that you share with this DNA Match will appear and go through them. Some you may know already and where they fit into your DNA and family so it will give you a clue which side the DNA Match you are baffled about may come from, some may have trees.