How do you tell which normals are flipped in and which are out?

[HansJakob (Pwet)]

I am using Blender 2.81.

Here the mesh looks ok in Blender

Here in Unity 2019 you see the flipped normals appear as transparent

[rigoletto]

Turn the option on to view the normals: Blender 2.8 How Show/Hide Face normals?

[Stan_Pancakes (Stanislav Blinov)]

To add to @rigoletto’s answer, option 2:

and option 3:

[HansJakob (Pwet)]

very helpful guys thanks you saved me some time!