In newer versions of Blender the previously available Tool Shelf buttons Flip Direction and Recalculate, typically used to flip inverted surfaces that made meshes appear inside-out, have been moved to the main Mesh menu upper-left of the 3D View. As the option is now context sensitive it only appears as a menu option in Edit Mode.
Design note: the Mesh menu is specific to Edit Mode and does not appear unless that particular editing context is active.
Flip Selected Faces
To access the option and/or correct an inside-out or inverted face, first select the errant mesh element in Edit Mode  (using vertex, edge or face) and from the Mesh menu upper-left click Normals  then Flip  from the options that appear – Mesh » Normals » Flip. Alternatively use Alt + N to access the Normals context menu then select Flip. The highlighted inverted face/s will immediately invert based on their current orientation.
Design note: when selecting errant faces using vertices or edges make sure all component elements are included to the degree that the inverted faces highlight.
As an alternative to selectively inverting faces, or re-orientating a large mixed selection so everything points in one direction, inwards or outwards, use Recalculate Outside (Shift + N) or Recalculate Inside (Shift + Ctrl + N) options. To do this and force the orientation in one direction or the other, in Edit Mode select the entire mesh (A) and access the Mesh menu. Here select Normals » Recalculate Outside to flip faces/selections outwards, or Normals » Recalculate Inside to invert selections inwards – Mesh » Normals » Recalculate Outside/Inside.
Design note: in previous versions of Blender Recalculate tended to orientate faces randomly depending on the predominance of inwards or outwards facing surfaces prior to use. For Blender 2.8+ this is no longer the case, Recalculate Inwards and Recalculate Outwards forceably re-orientate selections based on the option selected, either in or out but not both.
For instances where surfaces are randomly facing one way or the other , or there are too many to select and manually process, use Recalculate Inwards/Outwards  to force everything to face the same direction, in or out, regardless – make a selection (individual, group or all) then click the Mesh menu, Normals then Recalculate Inwards/Outwards.
To assist determining which way a face might be pointing, especially if Backface Culling is not disabled in Viewport Overlays or Viewport Shading options (depending on the viewport display mode) or a given materials Settings properties, Blender 2.8+ includes an option to colour tint faces based on their orientation, RED for surfaces pointing inwards and BLUE for those pointing outwards, making it much easier to see at a glance what’s going on, which then allows for individual faces to be flipped or recalculated as needed. To use, click the Overlays button upper-right of the 3D View  and from the menu select Face Orientation  – Overlays » Face Orientation. Everything will immediately tint blue/red based on their orientation. To disable, repeat, select Overlays » Face Orientation.
Design note: if X-Ray (Alt + Z) is active for any of the Viewport Shading modes, Wireframe, Solid etc., Face Orientation won’t work.
If Backface Culling is disabled (off) it can be difficult to determine which way a given surfaces is pointing because both front and back of the mesh are the same. In this situation using the Overlays  setting Face Orientation  can help as it tints surfaces based on their orientation, inwards or outwards.