I think it would have to be several silvers in a box to make an appreciable difference. Also silver coins are actually heavier than clad.
A 90% weighs about 12.5 Grams
A 40% weighs about 11.5 Grams
A clad weighs about 11.3 Grams
Although there is a tollerance for the planchets and wear would also factor in... Probably hard to judge by weight for a box. Maybe for a roll....