Though it's possible for toxins like lead, cadmium and arsenic to be found in dyes and glazes used on stoneware, they have been pretty much eradicated in cookware sold in the US market. If they are present, they're found only on the exterior and not on the cooking surface, and the amounts are extremely tiny--well below any unsafe exposure levels.
Our recommendation is to buy known, reputable brands of stoneware cookware and not worry about it.