I agree with Matt - a dead giveaway is the "wallowed out" upper casting hole. You can see the plastic deformation (little ridge at the edge of the hole). ABS is too brittle to do that.

The rudder's also got that waxy, semi-translucent look to it that's typical of Lexan. ABS rudders look like PVC pipe - solid white (or yellowed white as the case may be).

BTW, the only place where Lexan rudders break is that corner by the upper casting hole. Once that breaks, the rudder's done.