In Sketch up you can determine the directions the normals are pointing. If an entities face is white, the normal is pointing away from the materials surface. If it is blue, not gray(that is shading) will be pointing toward the material. You always want the surface( face) normals to point away from the material.
There are two ways to correct face normals in SU. If an entity(face) is blue, and you want it corrected, select the face, right click/reverse face.
If all of the face normals are pointing in the wrong direction, select an entity, reverse its' face, then click orient faces All face normals will have the same orientation.
For fun I attached a SU file with a model of box with a wall thickness. 6 faces(entities) have normals facing the wrong direction. It is your mission to find, and correct them. Something fun to practice with.
And all of your models entities that you can view rotating the model should be white. Especially if you use the outer shell function for components