Calipers and a mirror mate, this way you keep a handle on the symetry. I also like to have a fairly good picture of the front and sides, the front one I often fold in half and hold infront of the sculpt to make sure it looks okay, then I double check using the mirror. It really does make things that look out of whack jump out at you. Looks like youve got your basic forms down. Making things really smooth will be your main problem I think though. You'll have to use differing sizes of rakes. big ones to start off with to break down any really major uneven lumps and bumps and then work down in steps refining as you go. Rake, smooth, rake smooth ....until you get a realy nice even finish. If you're using water clay foam sponges are good too to smooth out the clay towards the final satges of breaking the surface down. Hope this helps mate.