Silhouettes
You can select the silhouette edge of a rounded surface, such as a cylinder.
To set
silhouette edge visibility, click Options
(Standard toolbar) or Tools, Options, and select Display/Selection,
Dynamic highlight from graphics view.
Silhouette edges are included in box-select operations.
To select
a silhouette edge:
Open a
sketch on a model face, plane, or in an active drawing view.
Select
one of the tools:
Convert Entities
Offset Entities
Smart
Dimensions 
Add
Relations 
Move the
pointer over a cylindrical face until the pointer changes to 
Select
the highlighted edge.