Arms: Trailing or Leading?

Spiral arms might be trailing (top left: convex part advances) or leading (lower right: concave part advances). From Doppler measurements we know which side approaches and which recedes. But to decide on whether the spiral is trailing or leading we also need to know whether the top part is behind or in front of the plane of the sky (i.e. whether we are looking down on the top of the disk, or up from underneath).