Confirming Mesh Connectivity

The mesh connectivity tool allows you to view free edges in the 3D view.

Free edges can be used to confirm if a mesh is connected.
Note: A free edge is an edge that is only on the boundary of a single face.
On the 3D View contextual tabs set, on the Mesh tab, in the Tools group, click the  Connectivity icon.

Figure 1. On the left, an example of two unconnected rectangles. To the right, the two rectangles are unioned. Edges displayed in red indicate free edges.