check the manual. You can detect using polygon collision and get the vertex of the model, by using vec_for_vertex you should be able to get exact positions of the vertex that was hit. You can also use normal to detect which way the normals were facing. It does have some complex poly collision if you choose to use it, but be wary, any collision detection in 3DGS takes alot of framerate if there are multiple objects using it, c_trace takes the most out of all the commands.