Render Boundaries

Select Mesh→Render→Boundaries...

Before rendering boundaries, make sure to follow these steps to get the labels defined correctly:

Now you can render the boundaries. Note that label generation for boundaries will only work correctly when the mesh has unambiguous endpoints in each direction. See below for examples that will work and those that will not.

The endpoints on these meshes are clear (the red dots):

The endpoints on these meshes are not clear: