It will be almost impossible to tell I suppose, other than observing the burrowing going minimal. I don't really think you can knock down the amount of crayfish unless you find a means to kill them in large quantities. They will out-breed your efforts. The only way I think it may work is to stock fish that work 24/7 to keep the numbers under control.
I only have planted lilies in my pond, the rest have come uninvited. I have no crayfish (yet), so I don't have a problem with too little plants.
There are some pretty good threads on this site about what plants to choose that don't take over a pond and look nice. I used to have a decently maintained pond garden until an 11yo boy gained priority. I used to just go to the local pond garden place and picked out plants that looked nice.
