Face Orientation in Blender
To assist determining which way a face might be pointing, especially if Backface Culling](Backface Culling – Blender Knowledgebase) 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.