It's as simple as drawing a line between them. Back in Polygon mode, the new triangles are selectable as Polygons, just as the quads are. Before leaving the subject of Polygon sub-object selection mode, we should consider the issue of normals. This ray is used to indicate the renderable side of the polygon. The concept of a perpendicular ray only makes sense for polygons that are perfectly planar.
Triangles are, inherently, perfectly planar.
Thus in Editable Mesh, it makes sense to be able to display the normals for the Faces in selected Polygons. The next image can be confusing to read, but note that although the top quad is not rendering, it still exists and its two normals are pointing downward. Editable Poly takes a simplified approach that makes sense in the overwhelming majority of cases. Normals cannot be displayed, nor can you flip the normals of selected polygons. You can only flip all the normals of a mesh Element as a group.