If a silver proof does exist, and assuming it is the same thickness as the cupro-nickel crown, it would be more easily identifiable by weight than by the unreliable "ring" test:
Cu-Ni: 28.27590 g (the standard weight given in the Coinage Act 1946)
.925 silver: 32.82 g
.999 silver: 33.16 g
Otherwise, a silver coin of the same weight as the cupro-nickel crown would be noticeably thinner (about 2.47 mm compared with 2.91 mm, so easily visible to the naked eye when placed side by side).
This is due to the differing densities of the metals/alloys: Ag 10.49 g/cm3, sterling silver 10.3845, cupro-nickel 8.9455.