visualize panel model

hi,I often perform flow calculations with so-called boundary element methods. These make use of 3D panel models describing the shape of an object in a similar way as the matlab patch function does using 'faces' and 'vertices'. I built something to visualize these models, logically using the matlab patch function. For each face of the patch I calculated the normal direction and now i want to indicate if the normal vector points inwards or outwards the model. Anyone a suggestion how to determine if a normal vector points in of outward a 3D model described by patches ? thanks!!