Select Sharp Edges from the dropdown menu of the
Detect tool in the
Indentify group of the
Prepare tab.
Enter a Max edge angle in
the options panel or keep the default value. This is the angle between
normals of faces that share an edge. Additionally, choose to find
Convex or Concave
sharps.
Click Find matching sharp
edges
to find edges sharper than the Max edge angle
specified in a faceted or solid body. Any edges with an edge angle greater
than the angle specified will be highlighted.
Alternatively, in the options panel, you can select
Automatically find matches, however, it may take a
little longer to search and display the results on large models.
Select the red highlighted face and choose to
convert it to a named selection or a primary selection .
or a primary selection 
.