It has to be likely not proven.
The breeder of the pups has to produce either a gun licence, a permission letter covering pest control, or proof of land ownership.
What bearing all that nonsense will have on the pups is beyond me. Fair enough, you can prove that the Dam is a working animal, but how the hell do they know if the pups are to be workers or not?
My JRT is legally docked, if I remember later on, I'll scan the paperwork, and post a copy up here just so people can see the bullshit. It's about 10 fecking pages long!
Out of interest how much do different people pay