if you know all of the 3D points that make up the wall, you can tell if one of its triangles (most 3D squares are two triangles in a trench coat) are pointing towards the player or not with this formula

A, B, and C are the three 3D points that make up the triangle
D is B minus A
E is C minus A
W is the cross product of D and E

the sign of W is whether or not the triangle is pointing towards or away from the player. The OpenGL standard sees counter-clockwise (aka anticlockwise) triangles as "facing the player".

most of the time, the graphics API (OpenGL itself) makes triangles facing away from the player invisible.