Whenever you have selected triangles or surfaces you can expand or shrink the selection in the context menu or by pressing Plus or Minus. If you expand the selection, all unselected triangles adjacent to the current selection are added to the selection. If you shrink the selection, all selected triangles adjacent to unselected triangles are removed from the selection.
If a part has holes, all surrounding triangles can be selected by right-clicking on the edge of a hole and choosing Select surrounding triangles. Alternatively, if you click on Select Attached Triangle Only. only the triangle to which the edge belongs is selected. That way, it is much easier to select tiny triangles along open edges, which may be created while working on the triangle mesh.
In the Actions register, the button Select Shells with Negative Volume enables the user to select all inverted shells. This function is very useful to select and then invert or remove those shells. However, it is only reliable for closed and oriented shells.